PL/SQL Tutorial - PL SQL Interview Questions

Sunday, November 29, 2015

Oracle Interview Questions - Complete Dump

Oracle Interview Questions

1. Explain the difference between a hot backup and a cold backup and the benefits associated with each.

A hot backup is basically taking a backup of the database while it is still up and running and it must be in archive log mode. A cold backup is taking a backup of the database while it is shut down and does not require being in archive log mode. The benefit of taking a hot backup is that the database is still available for use while the backup is occurring and you can recover the database to any point in time. The benefit of taking a cold backup is that it is typically easier to administer the backup and recovery process. In addition, since you are taking cold backups the database does not require being in archive log mode and thus there will be a slight performance gain as the database is not cutting archive logs to disk.

2. You have just had to restore from backup and do not have any control files. How would you go about bringing up this database?

I would create a text based backup control file, stipulating where on disk all the data files were and then issue the recover command with the using backup control file clause.

3. How do you switch from an init.ora file to a spfile?

Issue the create spfile from pfile command.

4. Explain the difference between a data block, an extent and a segment.

A data block is the smallest unit of logical storage for a database object. As objects grow they take chunks of additional storage that are composed of contiguous data blocks. These groupings of contiguous data blocks are called extents. All the extents that an object takes when grouped together are considered the segment of the database object.

5. Give two examples of how you might determine the structure of the table DEPT.

Use the describe command or use the dbms_metadata.get_ddl package.

6. Where would you look for errors from the database engine?

In the alert log.

7. Compare and contrast TRUNCATE and DELETE for a table.

Both the truncate and delete command have the desired outcome of getting rid of all the rows in a table. The difference between the two is that the truncate command is a DDL operation and just moves the high water mark and produces few rollback data. The delete command, on the other hand, is a DML operation, which will produce rollback data and thus take longer to complete.

8. Give the reasoning behind using an index.

Faster access to data blocks in a table.

9. Give the two types of tables involved in producing a star schema and the type of data they hold.

Fact tables and dimension tables. A fact table contains measurements while dimension tables will contain data that will help describe the fact tables.

10. What type of index should you use on a fact table?

A Bitmap index.

11. Give some examples of the types of database constraints you may find in Oracle and indicate their purpose.

A Primary or Unique Key can be used to enforce uniqueness on one or more columns.
A Referential Integrity constraint can be used to enforce a Foreign Key relationship between two tables.
A Not Null constraint - to ensure a value is entered in a column
A Value constraint - to check a column value against a specific set of values.

12. A table is classified as a parent table and you want to drop and re-create it. How would you do this without affecting the children tables?

Disable the foreign key constraint to the parent, drop the table, re-create the table, enable the foreign key constraint.

13. Explain the difference between ARCHIVELOG mode and NOARCHIVELOG mode and the benefits and disadvantages to each.

ARCHIVELOG mode is a mode that you can put the database in for creating a backup of all transactions that have occurred in the database so that you can recover to any point in time. NOARCHIVELOG mode is basically the absence of ARCHIVELOG mode and has the disadvantage of not being able to recover to any point in time. NOARCHIVELOG mode does have the advantage of not having to write transactions to an archive log and thus increases the performance of the database slightly.

14. What command would you use to create a backup control file?

Alter database backup control file to trace.

15. Give the stages of instance startup to a usable state where normal users may access it.

STARTUP NOMOUNT - Instance startup

STARTUP MOUNT - The database is mounted

STARTUP OPEN - The database is opened

16. What column differentiates the V$ views to the GV$ views and how?

The INST_ID column which indicates the instance in a RAC environment the information came from.

17. How would you go about generating an EXPLAIN plan?

Create a plan table with utlxplan.sql.

Use the explain plan set statement_id = 'tst1' into plan_table for a SQL statement

Look at the explain plan with utlxplp.sql or utlxpls.sql

18. How would you go about increasing the buffer cache hit ratio?

Use the buffer cache advisory over a given workload and then query the v$db_cache_advice table. If a change was necessary then I would use the alter system set db_cache_size command.

19. Explain an ORA-01555.

You get this error when you get a snapshot too old within rollback. It can usually be solved by increasing the undo retention or increasing the size of rollbacks. You should also look at the logic involved in the application getting the error message.

20. Explain the difference between $ORACLE_HOME and $ORACLE_BASE.

ORACLE_BASE is the root directory for oracle. ORACLE_HOME located beneath ORACLE_BASE is where the oracle products reside.

Oracle Interview Questions

1. How would you determine the time zone under which a database was operating?

SELECT dbtimezone FROM DUAL;

2. Explain the use of setting GLOBAL_NAMES equal to TRUE.

It ensure the use of consistent naming conventions for databases and links in a networked environment.

3. What command would you use to encrypt a PL/SQL application?

WRAP

4. Explain the difference between a FUNCTION, PROCEDURE and PACKAGE.

They are all named PL/SQL blocks.

Function must return a value. Can be called inside a query.

Procedure may or may not return value.

Package is the collection of functions, procedures, variables which can be logically grouped together.

5. Explain the use of table functions.

6. Name three advisory statistics you can collect.

7. Where in the Oracle directory tree structure are audit traces placed?

8. Explain materialized views and how they are used.

9. When a user process fails, what background process cleans up after it?

PMON

10. What background process refreshes materialized views?

Job Queue Process (CJQ)

11. How would you determine what sessions are connected and what resources they are waiting for?

v$session,v$session_wait

12. Describe what redo logs are.

13. How would you force a log switch?

alter system switch logfile;

14. Give two methods you could use to determine what DDL changes have been made.

15. What does coalescing a tablespace do?

Coalesce simply takes contigous free extents and makes them into a single bigger free extent.

16. What is the difference between a TEMPORARY tablespace and a PERMANENT tablespace?

TEMP tablespace gets cleared once the transaction is done where as PERMANENT tablespace retails the data.

17. Name a tablespace automatically created when you create a database.

SYSTEM

18. When creating a user, what permissions must you grant to allow them to connect to the database?

Grant create session to username;

19. How do you add a data file to a tablespace?

alter tablespace USERS add datafile '/ora01/oradata/users02.dbf' size 50M;

20. How do you resize a data file?

alter database datafile '/ora01/oradata/users02.dbf' resize 100M;

21. What view would you use to look at the size of a data file?

dba_data_files

22. What view would you use to determine free space in a tablespace?

dba_free_space

23. How would you determine who has added a row to a table?

By implementing an INSERT trigger for logging details during each INSERT operation on the table

24. How can you rebuild an index?

ALTER INDEX index_name REBUILD;

25. Explain what partitioning is and what its benefit is.

A table partition is also a table segment, and by using partitioning technique we can enhance performance of table access.

26. You have just compiled a PL/SQL package but got errors, how would you view the errors?

show errors

27. How can you gather statistics on a table?

exec dbms_stats.gather_table_stats

Also, remember to analyze all associated indexes on that table using dbms_stats.gather_index_stats

28. How can you enable a trace for a session?

alter session set sql_trace='TRUE';

29. What is the difference between the SQL*Loader and IMPORT utilities?

SQL*LOADER loads external data which is in OS files to oracle database tables while IMPORT utility imports data only which is exported by EXPORT utility of oracle database.

30. Name two files used for network connection to a database.

TNSNAMES.ORA and SQLNET.ORA

Oracle Interview Questions

1. Describe the difference between a procedure, function and anonymous pl/sql block. Candidate should mention use of DECLARE statement, a function must return a value while a procedure doesn't have to.

2. What is a mutating table error and how can you get around it? This happens with triggers. It occurs because the trigger is trying to update a row it is currently using. The usual fix involves either use of views or temporary tables so the database is selecting from one while updating the other.

3. Describe the use of %ROWTYPE and %TYPE in PL/SQL Expected answer: %ROWTYPE allows you to associate a variable with an entire table row. The %TYPE associates a variable with a single column type.

4. What packages (if any) has Oracle provided for use by developers? Expected answer: Oracle provides the DBMS_ series of packages. There are many which developers should be aware of such as DBMS_SQL, DBMS_PIPE, DBMS_TRANSACTION, DBMS_LOCK, DBMS_ALERT, DBMS_OUTPUT, DBMS_JOB, DBMS_UTILITY, DBMS_DDL, UTL_FILE. If they can mention a few of these and describe how they used them, even better. If they include the SQL routines provided by Oracle, great, but not really what was asked.

5. Describe the use of PL/SQL tables Expected answer: PL/SQL tables are scalar arrays that can be referenced by a binary integer. They can be used to hold values for use in later queries or calculations. In Oracle 8 they will be able to be of the %ROWTYPE designation, or RECORD.

6. When is a declare statement needed ? The DECLARE statement is used in PL/SQL anonymous blocks such as with stand alone, non-stored PL/SQL procedures. It must come first in a PL/SQL stand alone file if it is used.

7. In what order should a open/fetch/loop set of commands in a PL/SQL block be implemented if you use the %NOTFOUND cursor variable in the exit when statement? Why? Expected answer: OPEN then FETCH then LOOP followed by the exit when. If not specified in this order will result in the final return being done twice because of the way the %NOTFOUND is handled by PL/SQL.

8. What are SQLCODE and SQLERRM and why are they important for PL/SQL developers? Expected answer: SQLCODE returns the value of the error number for the last error encountered. The SQLERRM returns the actual error message for the last error encountered. They can be used in exception handling to report, or, store in an error log table, the error that occurred in the code. These are especially useful for the WHEN OTHERS exception.

9. How can you find within a PL/SQL block, if a cursor is open? Expected answer: Use the %ISOPEN cursor status variable.

10. How can you generate debugging output from PL/SQL? Expected answer: Use the DBMS_OUTPUT package. Another possible method is to just use the SHOW ERROR command, but this only shows errors. The DBMS_OUTPUT package can be used to show intermediate results from loops and the status of variables as the procedure is executed. The new package UTL_FILE can also be used.

11. What are the types of triggers? Expected Answer: There are 12 types of triggers in PL/SQL that consist of combinations of the BEFORE, AFTER, ROW, TABLE, INSERT, UPDATE, DELETE and ALL key words: BEFORE ALL ROW INSERT AFTER ALL ROW INSERT BEFORE INSERT AFTER INSERT etc.

Oracle Interview Questions

1. A tablespace has a table with 30 extents in it. Is this bad? Why or why not.

Multiple extents in and of themselves aren?t bad. However if you also have chained rows this can hurt performance.

2. How do you set up tablespaces during an Oracle installation?

You should always attempt to use the Oracle Flexible Architecture standard or another partitioning scheme to ensure proper separation of SYSTEM, ROLLBACK, REDO LOG, DATA, TEMPORARY and INDEX segments.

3. You see multiple fragments in the SYSTEM tablespace, what should you check first?

Ensure that users don?t have the SYSTEM tablespace as their TEMPORARY or DEFAULT tablespace assignment by checking the DBA_USERS view.

4. What are some indications that you need to increase the SHARED_POOL_SIZE parameter?

Poor data dictionary or library cache hit ratios, getting error ORA-04031. Another indication is steadily decreasing performance with all other tuning parameters the same.

5. What is the general guideline for sizing db_block_size and db_multi_block_read for an application that does many full table scans?

Oracle almost always reads in 64k chunks. The two should have a product equal to 64 or a multiple of 64.

6. What is the fastest query method for a table

Fetch by rowid

7. Explain the use of TKPROF? What initialization parameter should be turned on to get full TKPROF output?

The tkprof tool is a tuning tool used to determine cpu and execution times for SQL statements. You use it by first setting timed_statistics to true in the initialization file and then turning on tracing for either the entire database via the sql_trace parameter or for the session using the ALTER SESSION command. Once the trace file is generated you run the tkprof tool against the trace file and then look at the output from the tkprof tool. This can also be used to generate explain plan output.

8. When looking at v$sysstat you see that sorts (disk) is high. Is this bad or good? If bad -How do you correct it?

If you get excessive disk sorts this is bad. This indicates you need to tune the sort area parameters in the initialization files. The major sort are parameter is the SORT_AREA_SIZe parameter.

9. When should you increase copy latches? What parameters control copy latches

When you get excessive contention for the copy latches as shown by the "redo copy" latch hit ratio. You can increase copy latches via the initialization parameter LOG_SIMULTANEOUS_COPIES to twice the number of CPUs on your system.

10. Where can you get a list of all initialization parameters for your instance? How about an indication if they are default settings or have been changed

You can look in the init.ora file for an indication of manually set parameters. For all parameters, their value and whether or not the current value is the default value, look in the v$parameter view.

11. Describe hit ratio as it pertains to the database buffers. What is the difference between instantaneous and cumulative hit ratio and which should be used for tuning

The hit ratio is a measure of how many times the database was able to read a value from the buffers verses how many times it had to re-read a data value from the disks. A value greater than 80-90% is good, less could indicate problems. If you simply take the ratio of existing parameters this will be a cumulative value since the database started. If you do a comparison between pairs of readings based on some arbitrary time span, this is the instantaneous ratio for that time span. Generally speaking an instantaneous reading gives more valuable data since it will tell you what your instance is doing for the time it was generated over.

12. Discuss row chaining, how does it happen? How can you reduce it? How do you correct it

Row chaining occurs when a VARCHAR2 value is updated and the length of the new value is longer than the old value and won?t fit in the remaining block space. This results in the row chaining to another block. It can be reduced by setting the storage parameters on the table to appropriate values. It can be corrected by export and import of the effected table.

Oracle Interview Questions

1. Give one method for transferring a table from one schema to another:

There are several possible methods, export-import, CREATE TABLE... AS SELECT, or COPY.

2. What is the purpose of the IMPORT option IGNORE? What is it?s default setting

The IMPORT IGNORE option tells import to ignore "already exists" errors. If it is not specified the tables that already exist will be skipped. If it is specified, the error is ignored and the tables data will be inserted. The default value is N.

3. You have a rollback segment in a version 7.2 database that has expanded beyond optimal, how can it be restored to optimal

Use the ALTER TABLESPACE ..... SHRINK command.

4. If the DEFAULT and TEMPORARY tablespace clauses are left out of a CREATE USER command what happens? Is this bad or good? Why

The user is assigned the SYSTEM tablespace as a default and temporary tablespace. This is bad because it causes user objects and temporary segments to be placed into the SYSTEM tablespace resulting in fragmentation and improper table placement (only data dictionary objects and the system rollback segment should be in SYSTEM).

5. What are some of the Oracle provided packages that DBAs should be aware of

Oracle provides a number of packages in the form of the DBMS_ packages owned by the SYS user. The packages used by DBAs may include: DBMS_SHARED_POOL, DBMS_UTILITY, DBMS_SQL, DBMS_DDL, DBMS_SESSION, DBMS_OUTPUT and DBMS_SNAPSHOT. They may also try to answer with the UTL*.SQL or CAT*.SQL series of SQL procedures. These can be viewed as extra credit but aren?t part of the answer.

6. What happens if the constraint name is left out of a constraint clause

The Oracle system will use the default name of SYS_Cxxxx where xxxx is a system generated number. This is bad since it makes tracking which table the constraint belongs to or what the constraint does harder.

7. What happens if a tablespace clause is left off of a primary key constraint clause

This results in the index that is automatically generated being placed in then users default tablespace. Since this will usually be the same tablespace as the table is being created in, this can cause serious performance problems.

8. What is the proper method for disabling and re-enabling a primary key constraint

You use the ALTER TABLE command for both. However, for the enable clause you must specify the USING INDEX and TABLESPACE clause for primary keys.

9. What happens if a primary key constraint is disabled and then enabled without fully specifying the index clause

The index is created in the user?s default tablespace and all sizing information is lost. Oracle doesn?t store this information as a part of the constraint definition, but only as part of the index definition, when the constraint was disabled the index was dropped and the information is gone.

10. (On UNIX) When should more than one DB writer process be used? How many should be used

If the UNIX system being used is capable of asynchronous IO then only one is required, if the system is not capable of asynchronous IO then up to twice the number of disks used by Oracle number of DB writers should be specified by use of the db_writers initialization parameter.

11. You are using hot backup without being in archivelog mode, can you recover in the event of a failure? Why or why not

You can?t use hot backup without being in archivelog mode. So no, you couldn?t recover.

12. What causes the "snapshot too old" error? How can this be prevented or mitigated

This is caused by large or long running transactions that have either wrapped onto their own rollback space or have had another transaction write on part of their rollback space. This can be prevented or mitigated by breaking the transaction into a set of smaller transactions or increasing the size of the rollback segments and their extents.

13. How can you tell if a database object is invalid By checking the status column of the DBA_, ALL_ or USER_OBJECTS views, depending upon whether you own or only have permission on the view or are using a DBA account.

13. A user is getting an ORA-00942 error yet you know you have granted them permission on the table, what else should you check

You need to check that the user has specified the full name of the object (select empid from scott.emp; instead of select empid from emp;) or has a synonym that balls to the object (create synonym emp for scott.emp;)

14. A developer is trying to create a view and the database won?t let him. He has the "DEVELOPER" role which has the "CREATE VIEW" system privilege and SELECT grants on the tables he is using, what is the problem

You need to verify the developer has direct grants on all tables used in the view. You can?t create a stored object with grants given through views.

15. If you have an example table, what is the best way to get sizing data for the production table implementation

The best way is to analyze the table and then use the data provided in the DBA_TABLES view to get the average row length and other pertinent data for the calculation. The quick and dirty way is to look at the number of blocks the table is actually using and ratio the number of rows in the table to its number of blocks against the number of expected rows.

16. How can you find out how many users are currently logged into the database? How can you find their operating system id

There are several ways. One is to look at the v$session or v$process views. Another way is to check the current_logins parameter in the v$sysstat view. Another if you are on UNIX is to do a "ps -ef|grep oracle|wc -l? command, but this only works against a single instance installation.

17. A user selects from a sequence and gets back two values, his select is: SELECT pk_seq.nextval FROM dual;What is the problem Somehow two values have been inserted into the dual table. This table is a single row, single column table that should only have one value in it.

18. How can you determine if an index needs to be dropped and rebuilt

Run the ANALYZE INDEX command on the index to validate its structure and then calculate the ratio of LF_BLK_LEN/LF_BLK_LEN+BR_BLK_LEN and if it isn?t near 1.0 (i.e. greater than 0.7 or so) then the index should be rebuilt. Or if the ratio BR_BLK_LEN/ LF_BLK_LEN+BR_BLK_LEN is nearing 0.3.

Oracle Interview Questions

1. How can variables be passed to a SQL routine

By use of the & symbol. For passing in variables the numbers 1-8 can be used (&1, &2,...,&8) to pass the values after the command into the SQLPLUS session. To be prompted for a specific variable, place the ampersanded variable in the code itself: "select * from dba_tables where owner=&owner_name;" . Use of double ampersands tells SQLPLUS to resubstitute the value for each subsequent use of the variable, a single ampersand will cause a reprompt for the value unless an ACCEPT statement is used to get the value from the user.

2. You want to include a carriage return/linefeed in your output from a SQL script, how can you do this

The best method is to use the CHR() function (CHR(10) is a return/linefeed) and the concatenation function "||". Another method, although it is hard to document and isn?t always portable is to use the return/linefeed as a part of a quoted string.

3. How can you call a PL/SQL procedure from SQL

By use of the EXECUTE (short form EXEC) command.

4. How do you execute a host operating system command from within SQL

By use of the exclamation ball "!" (in UNIX and some other OS) or the HOST (HO) command.

5. You want to use SQL to build SQL, what is this called and give an example

This is called dynamic SQL. An example would be: set lines 90 pages 0 termout off feedback off verify off spool drop_all.sql select ?drop user ?||username||? cascade;? from dba_users where username not in ("SYS?,?SYSTEM?); spool off Essentially you are looking to see that they know to include a command (in this case DROP USER...CASCADE;) and that you need to concatenate using the ?||? the values selected from the database.

6. What SQLPlus command is used to format output from a select

This is best done with the COLUMN command.

7. You want to group the following set of select returns, what can you group on

Max(sum_of_cost), min(sum_of_cost), count(item_no), item_no The only column that can be grouped on is the "item_no" column, the rest have aggregate functions associated with them.

8. What special Oracle feature allows you to specify how the cost based system treats a SQL statement

The COST based system allows the use of HINTs to control the optimizer path selection. If they can give some example hints such as FIRST ROWS, ALL ROWS, USING INDEX, STAR, even better.

9. You want to determine the location of identical rows in a table before attempting to place a unique index on the table, how can this be done

Oracle tables always have one guaranteed unique column, the rowid column. If you use a min/max function against your rowid and then select against the proposed primary key you can squeeze out the rowids of the duplicate rows pretty quick. For example: select rowid from emp e where e.rowid > (select min(x.rowid) from emp x where x.emp_no = e.emp_no); In the situation where multiple columns make up the proposed key, they must all be used in the where clause.

10. What is a Cartesian product

A Cartesian product is the result of an unrestricted join of two or more tables. The result set of a three table Cartesian product will have x * y * z number of rows where x, y, z correspond to the number of rows in each table involved in the join.

11. You are joining a local and a remote table, the network manager complains about the traffic involved, how can you reduce the network traffic Push the processing of the remote data to the remote instance by using a view to pre-select the information for the join. This will result in only the data required for the join being sent across.

11. What is the default ordering of an ORDER BY clause in a SELECT statement

Ascending

12. What is tkprof and how is it used

13. What is explain plan and how is it used

The EXPLAIN PLAN command is a tool to tune SQL statements. To use it you must have an explain_table generated in the user you are running the explain plan for. This is created using the utlxplan.sql script. Once the explain plan table exists you run the explain plan command giving as its argument the SQL statement to be explained. The explain_plan table is then queried to see the execution plan of the statement. Explain plans can also be run using tkprof.

14. How do you set the number of lines on a page of output? The width

The SET command in SQLPLUS is used to control the number of lines generated per page and the width of those lines, for example SET PAGESIZE 60 LINESIZE 80 will generate reports that are 60 lines long with a line width of 80 characters. The PAGESIZE and LINESIZE options can be shortened to PAGES and LINES.

15. How do you prevent output from coming to the screen

The SET option TERMOUT controls output to the screen. Setting TERMOUT OFF turns off screen output. This option can be shortened to TERM.

16. How do you prevent Oracle from giving you informational messages during and after a SQL statement execution

The SET options FEEDBACK and VERIFY can be set to OFF.

17. How do you generate file output from SQL

By use of the SPOOL comm

Oracle Interview Questions

1. What is a CO-RELATED SUBQUERY

A CO-RELATED SUBQUERY is one that has a correlation name as table or view designator in the FROM clause of the outer query and the same correlation name as a qualifier of a search condition in the WHERE clause of the subquery. eg

    SELECT  field1 from table1 X
    WHERE  field2>(select avg(field2) from table1 Y
                                      where
                                      field1=X.field1);

(The subquery in a correlated subquery is revaluated for every row of the table or view named in the outer query.)

2. What are various joins used while writing SUBQUERIES

Self join-Its a join foreign key of a table references the same table.

Outer Join--Its a join condition used where One can query all the rows of one of the tables in the join condition even though they don't satisfy the join condition.

Equi-join--Its a join condition that retrieves rows from one or more tables in which one or more columns in one table are equal to one or more columns in the second table.

3. What are various constraints used in SQL

NULL NOT NULL CHECK DEFAULT

4. What are different Oracle database objects

TABLES VIEWS INDEXES SYNONYMS SEQUENCES TABLESPACES etc

5. What is difference between Rename and Alias

Rename is a permanent name given to a table or column whereas Alias is a temporary name given to a table or column which do not exist once the SQL statement is executed.

6. What is a view

A view is stored procedure based on one or more tables, its a virtual table.

7. What are various privileges that a user can grant to another user

SELECT CONNECT RESOURCE

8. What is difference between UNIQUE and PRIMARY KEY constraints

A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys. The columns that compose PK are automatically define NOT NULL, whereas a column that compose a UNIQUE is not automatically defined to be mandatory must also specify the column is NOT NULL.

9. Can a primary key contain more than one columns

Yes

10. How you will avoid duplicating records in a query

By using DISTINCT

11. What is difference between SQL and SQL*PLUS

SQL*PLUS is a command line tool where as SQL and PL/SQL language interface and reporting tool. Its a command line tool that allows user to type SQL commands to be executed directly against an Oracle database. SQL is a language used to query the relational database(DML,DCL,DDL). SQL*PLUS commands are used to format query result, Set options, Edit SQL commands and PL/SQL.

12. Which datatype is used for storing graphics and images

LONG RAW data type is used for storing BLOB's (binary large objects).

13. How will you delete duplicating rows from a base table

DELETE FROM table_name A WHERE rowid>(SELECT min(rowid) from table_name B where B.table_no=A.table_no);

CREATE TABLE new_table AS SELECT DISTINCT * FROM old_table;

DROP old_table RENAME new_table TO old_table DELETE FROM table_name A WHERE rowid NOT IN (SELECT MAX(ROWID) FROM table_name GROUP BY column_name)

14. What is difference between SUBSTR and INSTR

SUBSTR returns a specified portion of a string eg SUBSTR('BCDEF',4) output BCDE INSTR provides character position in which a pattern is found in a string.

eg INSTR('ABC-DC-F','-',2) output 7 (2nd occurence of '-')

15. There is a string '120000 12 0 .125' ,how you will find the position of the decimal place

INSTR('120000 12 0 .125','.',1) output 13

16. There is a '%' sign in one field of a column. What will be the query to find it.

'\' Should be used before '%'.

17. When you use WHERE clause and when you use HAVING clause

HAVING clause is used when you want to specify a condition for a group function and it is written after GROUP BY clause The WHERE clause is used when you want to specify a condition for columns, single row functions except group functions and it is written before GROUP BY clause if it is used.

18. Which is more faster - IN or EXISTS

EXISTS is more faster than IN because EXISTS returns a Boolean value whereas IN returns a value.

Appropriate answer will be....

Result of the subquery is small Then "IN" is typicaly more appropriate. and Result of the subquery is big/large/long Then "EXIST" is more appropriate.

19. What is a OUTER JOIN

Outer Join--Its a join condition used where you can query all the rows of one of the tables in the join condition even though they dont satisfy the join condition.

20. How you will avoid your query from using indexes

SELECT * FROM emp Where emp_no+' '=12345;

i.e you have to concatenate the column name with space within codes in the where condition.

SELECT /*+ FULL(a) */ ename, emp_no from emp where emp_no=1234;

i.e using HINTS

Oracle Interview Questions

1. What is a pseudo column. Give some examples

It is a column that is not an actual column in the table.

eg USER, UID, SYSDATE, ROWNUM, ROWID, NULL, AND LEVEL.

Suppose customer table is there having different columns like customer no, payments.What will be the query to select top three max payments.

For top N queries, see http://www.orafaq.com/forum/mv/msg/160920/472554/102589/#msg_472554 post

2. What is the purpose of a cluster.

Oracle does not allow a user to specifically locate tables, since that is a part of the function of the RDBMS. However, for the purpose of increasing performance, oracle allows a developer to create a CLUSTER. A CLUSTER provides a means for storing data from different tables together for faster retrieval than if the table placement were left to the RDBMS.

3. What is a cursor.

Oracle uses work area to execute SQL statements and store processing information PL/SQL construct called a cursor lets you name a work area and access its stored information A cursor is a mechanism used to fetch more than one row in a Pl/SQl block.

4. Difference between an implicit & an explicit cursor.

PL/SQL declares a cursor implicitly for all SQL data manipulation statements, including quries that return only one row. However,queries that return more than one row you must declare an explicit cursor or use a cursor FOR loop.

Explicit cursor is a cursor in which the cursor name is explicitly assigned to a SELECT statement via the CURSOR...IS statement. An implicit cursor is used for all SQL statements Declare, Open, Fetch, Close. An explicit cursors are used to process multirow SELECT statements An implicit cursor is used to process INSERT, UPDATE, DELETE and single row SELECT. .INTO statements.

5. What are cursor attributes

%ROWCOUNT %NOTFOUND %FOUND %ISOPEN

6. What is a cursor for loop.

Cursor For Loop is a loop where oracle implicitly declares a loop variable, the loop index that of the same record type as the cursor's record.

7. Difference between NO DATA FOUND and %NOTFOUND

NO DATA FOUND is an exception raised only for the SELECT....INTO statements when the where clause of the querydoes not match any rows. When the where clause of the explicit cursor does not match any rows the %NOTFOUND attribute is set to TRUE instead.

8. What a SELECT FOR UPDATE cursor represent.

SELECT......FROM......FOR......UPDATE[OF column-reference][NOWAIT] The processing done in a fetch loop modifies the rows that have been retrieved by the cursor. A convenient way of modifying the rows is done by a method with two parts: the FOR UPDATE clause in the cursor declaration, WHERE CURRENT OF CLAUSE in an UPDATE or declaration statement.

9. What 'WHERE CURRENT OF ' clause does in a cursor.

LOOP

                         SELECT  num_credits  INTO  v_numcredits  FROM classes
                         WHERE  dept=123 and course=101;
                         UPDATE  students
                         SET current_credits=current_credits+v_numcredits
                         WHERE  CURRENT OF  X;
                         END  LOOP
                         COMMIT;
                         END;

10. What is use of a cursor variable? How it is defined.

A cursor variable is associated with different statements at run time, which can hold different values at run time. Static cursors can only be associated with one run time query. A cursor variable is reference type(like a pointer in C). Declaring a cursor variable: TYPE type_name IS REF CURSOR RETURN return_type type_name is the name of the reference type,return_type is a record type indicating the types of the select list that will eventually be returned by the cursor variable.

11. What should be the return type for a cursor variable.Can we use a scalar data type as return type.

The return type for a cursor must be a record type.It can be declared explicitly as a user-defined or %ROWTYPE can be used. eg TYPE t_studentsref IS REF CURSOR RETURN students%ROWTYPE

12. How you open and close a cursor variable.Why it is required.

OPEN cursor variable FOR SELECT...Statement CLOSE cursor variable In order to associate a cursor variable with a particular SELECT statement OPEN syntax is used.In order to free the resources used for the query CLOSE statement is used.

13. How you were passing cursor variables in PL/SQL 2.2.

In PL/SQL 2.2 cursor variables cannot be declared in a package.This is because the storage for a cursor variable has to be allocated using Pro*C or OCI with version 2.2,the only means of passing a cursor variable to a PL/SQL block is via bind variable or a procedure parameter.

14. Can cursor variables be stored in PL/SQL tables.If yes how.If not why.

No, a cursor variable points a row which cannot be stored in a two-dimensional PL/SQL table.

15. Difference between procedure and function.

Functions are named PL/SQL blocks that return a value and can be called with arguments procedure a named block that can be called with parameter. A procedure all is a PL/SQL statement by itself, while a Function call is called as part of an expression.

16. What are different modes of parameters used in functions and procedures.

IN OUT IN OUT

17. What is difference between a formal and an actual parameter

The variables declared in the procedure and which are passed, as arguments are called actual, the parameters in the procedure declaration. Actual parameters contain the values that are passed to a procedure and receive results. Formal parameters are the placeholders for the values of actual parameters

18. Can the default values be assigned to actual parameters.

Yes

19. Can a function take OUT parameters.If not why.

Yes. A function return a value, but can also have one or more OUT parameters. it is best practice, however to use a procedure rather than a function if you have multiple values to return.

20. What is syntax for dropping a procedure and a function .Are these operations possible.

              Drop Procedure procedure_name
              Drop Function function_name

21. What are ORACLE PRECOMPILERS.

Using ORACLE PRECOMPILERS ,SQL statements and PL/SQL blocks can be contained inside 3GL programs written in C,C++,COBOL,PASCAL, FORTRAN,PL/1 AND ADA. The Precompilers are known as Pro*C,Pro*Cobol,... This form of PL/SQL is known as embedded pl/sql,the language in which pl/sql is embedded is known as the host language. The prcompiler translates the embedded SQL and pl/sql ststements into calls to the precompiler runtime library.The output must be compiled and linked with this library to creater an executable.

22. What is OCI. What are its uses.

Oracle Call Interface is a method of accesing database from a 3GL program. Uses--No precompiler is required,PL/SQL blocks are executed like other DML statements.

                     The OCI library provides
                    -functions to parse SQL statemets
                    -bind input variables
                    -bind output variables
                    -execute statements
                    -fetch the results

23. Difference between database triggers and form triggers.

a) Data base trigger(DBT) fires when a DML operation is performed on a data base table.Form trigger(FT) Fires when user presses a key or navigates between fields on the screen b) Can be row level or statement level No distinction between row level and statement level. c) Can manipulate data stored in Oracle tables via SQL Can manipulate data in Oracle tables as well as variables in forms. d) Can be fired from any session executing the triggering DML statements. Can be fired only from the form that define the trigger. e) Can cause other database triggers to fire.Can cause other database triggers to fire,but not other form triggers.

24. What is an UTL_FILE.What are different procedures and functions associated

with it. UTL_FILE is a package that adds the ability to read and write to operating system files Procedures associated with it are FCLOSE, FCLOSE_ALL and 5 procedures to output data to a file PUT, PUT_LINE, NEW_LINE, PUTF, FFLUSH.PUT, FFLUSH.PUT_LINE,FFLUSH.NEW_LINE. Functions associated with it are FOPEN, ISOPEN.

25. Can you use a commit statement within a database trigger.

No (unless you make the trigger code an AT [AUTONOMOUS TRANSACTION] something you should never do)

26. What is the maximum buffer size that can be specified using the DBMS_OUTPUT.ENABLE function?

1,000,000

Oracle Interview Questions

1. When looking at the estat events report you see that you are getting busy buffer waits. Is this bad? How can you find what is causing it

Buffer busy waits could indicate contention in redo, rollback or data blocks. You need to check the v$waitstat view to see what areas are causing the problem. The value of the "count" column tells where the problem is, the "class" column tells you with what. UNDO is rollback segments, DATA is data base buffers.

2. If you see contention for library caches how can you fix it

Increase the size of the shared pool.

3. If you see statistics that deal with "undo" what are they really talking about

Rollback segments and associated structures.

4. If a tablespace has a default pctincrease of zero what will this cause (in relationship to the smon process)

The SMON process won?t automatically coalesce its free space fragments.

5. If a tablespace shows excessive fragmentation what are some methods to defragment the tablespace? (7.1,7.2 and 7.3 only)

In Oracle 7.0 to 7.2 The use of the 'alter session set events 'immediate trace name coalesce level ts#';? command is the easiest way to defragment contiguous free space fragmentation. The ts# parameter corresponds to the ts# value found in the ts$ SYS table. In version 7.3 the ?alter tablespace coalesce;? is best. If the free space isn?t contiguous then export, drop and import of the tablespace contents may be the only way to reclaim non-contiguous free space.

6. How can you tell if a tablespace has excessive fragmentation

If a select against the dba_free_space table shows that the count of a tablespaces extents is greater than the count of its data files, then it is fragmented.

7. You see the following on a status report: redo log space requests 23 redo log space wait time 0 Is this something to worry about? What if redo log space wait time is high? How can you fix this Since the wait time is zero, no. If the wait time was high it might indicate a need for more or larger redo logs.

8. What can cause a high value for recursive calls? How can this be fixed

A high value for recursive calls is cause by improper cursor usage, excessive dynamic space management actions, and or excessive statement re-parses. You need to determine the cause and correct it By either relinking applications to hold cursors, use proper space management techniques (proper storage and sizing) or ensure repeat queries are placed in packages for proper reuse.

9. If you see a pin hit ratio of less than 0.8 in the estat library cache report is this a problem? If so, how do you fix it

This indicate that the shared pool may be too small. Increase the shared pool size.

10. If you see the value for reloads is high in the estat library cache report is this a matter for concern

Yes, you should strive for zero reloads if possible. If you see excessive reloads then increase the size of the shared pool.

11. You look at the dba_rollback_segs view and see that there is a large number of shrinks and they are of relatively small size, is this a problem? How can it be fixed if it is a problem

A large number of small shrinks indicates a need to increase the size of the rollback segment extents. Ideally you should have no shrinks or a small number of large shrinks. To fix this just increase the size of the extents and adjust optimal accordingly.

12. You look at the dba_rollback_segs view and see that you have a large number of wraps is this a problem

A large number of wraps indicates that your extent size for your rollback segments are probably too small. Increase the size of your extents to reduce the number of wraps. You can look at the average transaction size in the same view to get the information on transaction size.

Oracle Interview Questions

1. You have just started a new instance with a large SGA on a busy existing server. Performance is terrible, what should you check for

The first thing to check with a large SGA is that it isn?t being swapped out.

2. What OS user should be used for the first part of an Oracle installation (on UNIX)

You must use root first.

3. When should the default values for Oracle initialization parameters be used as is

Never

4. How many control files should you have? Where should they be located

At least 2 on separate disk spindles. Be sure they say on separate disks, not just file systems.

5. How many redo logs should you have and how should they be configured for maximum recoverability

You should have at least three groups of two redo logs with the two logs each on a separate disk spindle (mirrored by Oracle). The redo logs should not be on raw devices on UNIX if it can be avoided.

6. You have a simple application with no "hot" tables (i.e. uniform IO and access requirements). How many disks should you have assuming standard layout for SYSTEM, USER, TEMP and ROLLBACK tablespaces

At least 7, see disk configuration answer above.

7. Describe third normal form

Something like: In third normal form all attributes in an entity are related to the primary key and only to the primary key

8. Is the following statement true or false:

"All relational databases must be in third normal form" False. While 3NF is good for logical design most databases, if they have more than just a few tables, will not perform well using full 3NF. Usually some entities will be denormalized in the logical to physical transfer process.

9. What is an ERD

An ERD is an Entity-Relationship-Diagram. It is used to show the entities and relationships for a database logical model.

10. Why are recursive relationships bad? How do you resolve them

A recursive relationship (one where a table relates to itself) is bad when it is a hard relationship (i.e. neither side is a "may" both are "must") as this can result in it not being possible to put in a top or perhaps a bottom of the table (for example in the EMPLOYEE table you couldn?t put in the PRESIDENT of the company because he has no boss, or the junior janitor because he has no subordinates). These type of relationships are usually resolved by adding a small intersection entity.

11. What does a hard one-to-one relationship mean (one where the relationship on both ends is "must")

Expected answer: This means the two entities should probably be made into one entity.

12. How should a many-to-many relationship be handled

By adding an intersection entity table

13. What is an artificial (derived) primary key? When should an artificial (or derived) primary key be used

A derived key comes from a sequence. Usually it is used when a concatenated key becomes too cumbersome to use as a foreign key.

Oracle Interview Questions

1. When should you consider denormalization

Whenever performance analysis indicates it would be beneficial to do so without compromising data integrity.

2. How can you determine if an Oracle instance is up from the operating system level

There are several base Oracle processes that will be running on multi-user operating systems, these will be smon, pmon, dbwr and lgwr. Any answer that has them using their operating system process showing feature to check for these is acceptable. For example, on UNIX a ps -ef|grep dbwr will show what instances are up.

3. Users from the PC clients are getting messages indicating : ORA-06114: (Cnct err, can't get err txt. See Servr Msgs & Codes Manual)

What could the problem be The instance name is probably incorrect in their connection string.

4. Users from the PC clients are getting the following error stack: ERROR: ORA-01034: ORACLE not available ORA-07318: smsget: open error when opening sgadef.dbf file. HP-UX Error: 2: No such file or directory

What is the probable cause The Oracle instance is shutdown that they are trying to access, restart the instance.

5. How can you determine if the SQLNET process is running for SQLNET V1? How about V2

For SQLNET V1 check for the existence of the orasrv process. You can use the command "tcpctl status" to get a full status of the V1 TCPIP server, other protocols have similar command formats. For SQLNET V2 check for the presence of the LISTENER process(s) or you can issue the command "lsnrctl status".

6. What file will give you Oracle instance status information? Where is it located

The alert.ora log. It is located in the directory specified by the background_dump_dest parameter in the v$parameter table.

7. Users aren?t being allowed on the system. The following message is received: ORA-00257 archiver is stuck. Connect internal only, until freed What is the problem The archive destination is probably full, backup the archive logs and remove them and the archiver will re-start.

8. Where would you look to find out if a redo log was corrupted assuming you are using Oracle mirrored redo logs

There is no message that comes to the SQLDBA or SRVMGR programs during startup in this situation, you must check the alert.log file for this information.

9. You attempt to add a datafile and get: ORA-01118: cannot add anymore datafiles: limit of 40 exceeded What is the problem and how can you fix it When the database was created the db_files parameter in the initialization file was set to 40. You can shutdown and reset this to a higher value, up to the value of MAX_DATAFILES as specified at database creation. If the MAX_DATAFILES is set to low, you will have to rebuild the control file to increase it before proceeding.

10. You look at your fragmentation report and see that smon hasn?t coalesced any of you tablespaces, even though you know several have large chunks of contiguous free extents. What is the problem

Check the dba_tablespaces view for the value of pct_increase for the tablespaces. If pct_increase is zero, smon will not coalesce their free space.

11. Your users get the following error: ORA-00055 maximum number of DML locks exceeded What is the problem and how do you fix it The number of DML Locks is set by the initialization parameter DML_LOCKS. If this value is set to low (which it is by default) you will get this error. Increase the value of DML_LOCKS. If you are sure that this is just a temporary problem, you can have them wait and then try again later and the error should clear.

12. You get a call from you backup DBA while you are on vacation. He has corrupted all of the control files while playing with the ALTER DATABASE BACKUP CONTROLFILE command. What do you do

As long as all datafiles are safe and he was successful with the BACKUP controlfile command you can do the following: CONNECT INTERNAL STARTUP MOUNT (Take any read-only tablespaces offline before next step ALTER DATABASE DATAFILE .... OFFLINE;) RECOVER DATABASE USING BACKUP CONTROLFILE ALTER DATABASE OPEN RESETLOGS; (bring read-only tablespaces back online) Shutdown and backup the system, then restart If they have a recent output file from the ALTER DATABASE BACKUP CONTROL FILE TO TRACE; command, they can use that to recover as well. If no backup of the control file is available then the following will be required: CONNECT INTERNAL STARTUP NOMOUNT CREATE CONTROL FILE .....; However, they will need to know all of the datafiles, logfiles, and settings for MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY, MAXDATAFILES for the database to use the command.

Oracle Interview Questions

1. How would you determine the time zone under which a database was operating? 2. Explain the use of setting GLOBAL_NAMES equal to TRUE. 3. What command would you use to encrypt a PL/SQL application? 4. Explain the difference between a FUNCTION, PROCEDURE and PACKAGE. 5. Explain the use of table functions. 6. Name three advisory statistics you can collect. 7. Where in the Oracle directory tree structure are audit traces placed? 8. Explain materialized views and how they are used. 9. When a user process fails, what background process cleans up after it? 10. What background process refreshes materialized views? 11. How would you determine what sessions are connected and what resources they are waiting for? 12. Describe what redo logs are. 13. How would you force a log switch? 14. Give two methods you could use to determine what DDL changes have been made. 15. What does coalescing a tablespace do? 16. What is the difference between a TEMPORARY tablespace and a PERMANENT tablespace? 17. Name a tablespace automatically created when you create a database. 18. When creating a user, what permissions must you grant to allow them to connect to the database? 19. How do you add a data file to a tablespace? 20. How do you resize a data file? 21. What view would you use to look at the size of a data file? 22. What view would you use to determine free space in a tablespace? 23. How would you determine who has added a row to a table? 24. How can you rebuild an index? 25. Explain what partitioning is and what its benefit is. 26. You have just compiled a PL/SQL package but got errors, how would you view the errors? 27. How can you gather statistics on a table? 28. How can you enable a trace for a session? 29. What is the difference between the SQL*Loader and IMPORT utilities? 30. Name two files used for network connection to a database.

Oracle Interview Questions

1. In a system with an average of 40 concurrent users you get the following from a query on rollback extents:

ROLLBACK CUR EXTENTS

--------------------------

R01 11
R02 8
R03 12
R04 9
SYSTEM 4

You have room for each to grow by 20 more extents each. Is there a problem? Should you take any action

No there is not a problem. You have 40 extents showing and an average of 40 concurrent users. Since there is plenty of room to grow no action is needed.

2. You see multiple extents in the temporary tablespace. Is this a problem

As long as they are all the same size this isn?t a problem. In fact, it can even improve performance since Oracle won?t have to create a new extent when a user needs one.

3. Define OFA.

OFA stands for Optimal Flexible Architecture. It is a method of placing directories and files in an Oracle system so that you get the maximum flexibility for future tuning and file placement.

4. How do you set up your tablespace on installation

The answer here should show an understanding of separation of redo and rollback, data and indexes and isolation os SYSTEM tables from other tables. An example would be to specify that at least 7 disks should be used for an Oracle installation so that you can place

SYSTEM tablespace on one
Redo logs on two (mirrored redo logs)
TEMPORARY tablespace on another,
ROLLBACK tablespace on another and
still have two for DATA and INDEXES.

They should indicate how they will handle archive logs and exports as well. As long as they have a logical plan for combining or further separation more or less disks can be specified.

5. What should be done prior to installing Oracle (for the OS and the disks)

adjust kernel parameters or OS tuning parameters in accordance with installation guide. Be sure enough contiguous disk space is available.

6. You have installed Oracle and you are now setting up the actual instance. You have been waiting an hour for the initialization script to finish, what should you check first to determine if there is a problem

Check to make sure that the archiver isn?t stuck. If archive logging is turned on during install a large number of logs will be created. This can fill up your archive log destination causing Oracle to stop to wait for more space.

7. When configuring SQLNET on the server what files must be set up

INITIALIZATION file, TNSNAMES.ORA file, SQLNET.ORA file

8. When configuring SQLNET on the client what files need to be set up

SQLNET.ORA, TNSNAMES.ORA

9. What must be installed with ODBC on the client in order for it to work with Oracle

SQLNET and PROTOCOL (for example: TCPIP adapter) layers of the transport programs.

General Oracle Questions

What Oracle products have you worked with?
What version of Oracle were you running?
Compare Oracle to any other database that you know. Why would you prefer to work on one and not on the other?

Oracle DBA Questions

Typical DBA questions: 1. How many databases and what sizes?

2. Did you use online or off-line backups? Why?

used both based on business needs.

3. If you have to advise a backup strategy for a new application, how would you approach it and what questions will you ask?

is down time allowed, and what is this being used for Prod,QA or Dev. how huge this Db gonna be in few months .. what type of data is being used in this DB etc..

4. If a customer calls you about a hanging database session, what will you do to resolve it?

first thing ...any errors , if not check for session status.. locks waits , disk io, explain plan .. etc

5. How many control files and redo logs should a database have? atleast

2 ctl's and 3 logs

Oracle SQL and PL/SQL Interview Questions Advanced

What is a cursor ? ( Basic)

- Name or handle to a private SQL area where Oracle parses and fetches query results.

How to control how many cursors are open ?(Intermediate)

- Set OPEN_CURSORS parameter in initialization parameters.

What is shared SQL ? (Intermediate)

-Oracle recognizes similar statements. The SQL area is used many times for similar statements.

What is Parsing ? (Intermediate)

- Syntax checking, privileges checking, allocating Private SQL Area.

What is the difference between anonymous blocks and stored procedures ? ( Basic)

- Anonymous block is compiled only when called.

- Stored procedure is compiled and stored in database with the dependency information as well.

- Former is PL/SQL code directly called from an application. Latter is stored in database.

- Former has declare statement.Latter doesn't.

What are the advantages of procedures ? ( Basic)

- Loaded once and used many times

- Performance better coz all SQL stmts are sent in one go from the application to the database

- Security ( no object privileges are given directly )

- Invoker's rights possible

- Data integrity, productivity

What are standalone procedures ? (Basic)

- Those that are not part of package

How is a PL/SQL program stored in database ? (Advanced)

- Parsed code is stored. It's called P-code

How is a PL/SQL program executed ?(Advanced)

- Prior to Oracle 9i, we have only bytecode and a virtual machine in the database runs it. Later versions have faster native code execution.

- PL/SQL engine is the main component that executes procedural stmt and passes the SQL to the SQL statement executor.

What are the advantages and disadvantages of DBMS_SQL ? (Intermediate)

- It has all the advantages of dynamic sql .. like runtime construction of sql, DDL statements can be executed.

- Its advantage over EXECUTE IMMEDIATE is it can Describe objects

- It's kind of bulky and difficult compared to EXECUTE IMMEDIATE.

What is a package spec and package body ? Why the separation ? ( Basic)

- Spec declares public constructs. Body defines public constructs, additionally declares and defines Private constructs

- Separation helps make development easier

- Dependency is simplified. You can modify body without invalidating dependent objects.

What are the advantages of Packages ? ( Basic)

- Encapsulation of code logic

- Privileges to objects can be controlled

- Loaded once into memory , used subsequently.

- Dependency simplified

- Public/private procs, functions, variables

How do you handle exceptions for bulk operations ? (Intermediate)

- Use the SAVE EXCEPTIONS clause ( FORALL index IN bound_clause SAVE EXCEPTIONS LOOP ... END LOOP )

- Use 'Exceptions When Others' to handle the exceptions

- SQL%BULK_EXCEPTIONS(i).ERROR_CODE,

SQL%BULK_EXCEPTIONS(i).ERROR_INDEX

SQL%BULK_EXCEPTIONS.COUNT

Tell some tips to avoid performance problems in PL/SQL. (Intermediate to Advanced)

- Use FORALL instead of FOR, and use BULK COLLECT to avoid looping many times

- Tune SQL statements to avoid CPU overhead

- Use NOCOPY for OUT and IN OUT if the original value need not be retained. Overhead of keeping a copy of OUT is avoided.

- Reorder conditional tests to put least expensive ones first

- Minimize datatype conversions => Assign data to exact same type variables

- Use PLS_INTEGER for computation intensive code. NUMBER, INTEGER maintain precision and scale but not optimized for performance as additional checks are made to maintain precision and scale.

- Do not use subtypes like POSITIVE, NATURAL, INTEGER as they have additional checks

- Use BINARY_FLOAT, BINARY_DOUBLE

- EXECUTE IMMEDIATE is faster than DBMS_SQL

How to know PL/SQL compile parameters ?(Advanced)

- SHOW PARAMETERS PLSQL

- ALL_PLSQL_OBJECT_SETTINGS

What is MERGE ?( Basic)

- Combination of INSERT and UPDATE

Tell some new features in PL/SQL in 10g (Intermediate to Advanced)

- Regular expression functions REGEXP_LIKE , REGEXP_INSTR, REGEXP_REPLACE, and REGEXP_SUBSTR

- Compile time warnings

- Conditional compilation

- Improvement to native compilation

- BINARY_INTEGER made similar to PLS_INTEGER

- INDICES OF , VALUES OF in FORALL lets you work on non-consecutive indices

- Quoting mechanism . Instead of quoting single quotes twice every time, give your own delimiter to go on using single quotes.

Ex: q'!I'm a string, you're a string.!'

- Flashback Query functions. SCN_TO_TIMESTAMP, TIMESTAMP_TO_SCN

- Implicit conversion between CLOB and NCLOB

- Improved Overloading

- New datatypes BINARY_FLOAT, BINARY_DOUBLE

- Global optimization enabled

- PLS_INTEGER range increased to 32bit

- DYNAMIC WRAP using DBMS_DDL

What is a sequence ? (Basic)

- A database object that offers high-speed access to an integer value

- Guaranteed to be unique (within that sequence).

-Used commonly to generate Primary key values

Thursday, October 15, 2015

New PL SQL Interview Questions - Entry Level Interview Questions for Database Developers

SQL PL/SQL Question 1

How to display row number with records?

Select rownum, ename from emp;

SQL PL/SQL Question 2

How to view version information in Oracle?

Select banner from v$version;

SQL PL/SQL Question 3

How to find the second highest salary in emp table?

select min(sal) from emp a
where 1 = (select count(*) from emp b where a.sal <>

SQL PL/SQL Question 4

How to delete the duplicate rows from a table?

create table t1 ( col1 int, col2 int, col3 char(1) );
insert into t1 values(1,50, ‘a’);
insert into t1 values(1,50, ‘b’);
insert into t1 values(1,89, ‘x’);
insert into t1 values(1,89, ‘y’);
insert into t1 values(1,89, ‘z’);
select * from t1;

Col1	Col2	Col2
1	50	a
1	50	b
2	89	x
2	89	y
2	89	z

delete from T1
where rowid <> ( select max(rowid)
from t1 b
where b.col1 = t1.col1
and b.col2 = t1.col2 ) 3 rows deleted.

select * from t1;

Col1	Col2	Col2
1	50	a
2	89	z

will do it.

SQL PL/SQL Question 5

How to select a row using indexes?

You have to specify the indexed columns in the WHERE clause of query.

SQL PL/SQL Question 6

How to select the first 5 characters of FIRSTNAME column of EMP table?

select substr(firstname,1,5) from emp

SQL PL/SQL Question 7

How to concatenate the firstname and lastname from emp table?

select firstname ‘ ‘ lastname from emp

SQL PL/SQL Question 8

What's the difference between a primary key and a unique key?

Primary key does not allow nulls, Unique key allow nulls.

SQL PL/SQL Question 9

What is a self join?

A self join joins a table to itself.

Example

SELECT a.last_name Employee, b.last_name Manager
FROM employees a, employees b
WHERE b.employee_id = a.manager_id;

SQL PL/SQL Question 10

What is a transaction and ACID?

Transaction - A transaction is a logical unit of work. It must be commited or rolled back.
ACID - Atomicity, Consistency, Isolation and Duralbility, these are properties of a transaction.

SQL PL/SQL Question 11

How to add a column to a table?

alter table t1 add sal number;
alter table t1 add middle_name varchar(20);

SQL PL/SQL Question 12

Is it possible for a table to have more than one foreign key ?
A table can have any number of foreign keys. It can have only one primary key .

SQl PL/SQL Question 13

How to display number value in words?

SQL> select sal, (to_char(to_date(sal,'j'), 'jsp')) from emp;

SQL PL/SQL Question 14

What is candidate key, alternate key, composite key.

Candidate Key A candidate key is one that can identify each row of a table uniquely. Generally a candidate key becomes the primary key of the table.
Alternate KeyIf the table has more than one candidate key, one of them will become the primary key, and the rest are called alternate keys.
Composite Key: - A key formed by combining at least two or more columns is called composite key.

SQL PL/SQL Question 15

What's the difference between DELETE TABLE and TRUNCATE TABLE commands? Explain drop command.

Both Delete and Truncate will leave the structure of the table. Drop will remove the structure also.

Example
If tablename is T1.
To remove all the rows from a table t1.
Delete t1
Truncate table t1
Drop table t1.
Truncate is fast as compared to Delete. DELETE will generate undo information, in case of rollback, but TRUNCATE will not.

Full Table scan and index fast scan read data blocks up to high water mark and truncate resets high water mark but delete does not.So full table scan after Delete will not improve but after truncate it will be fast.
Delete is DML. Because truncate is a DDL, it performs implicit commit. You cannot rollback a truncate. Any uncommitted DML changes will also be committed with the TRUNCATE.
You cannot specify a WHERE clause in the TRUNCATE statement, but you can specify that in Delete.
When you truncate a table the storage for the table and all the indexes can be reset back to its initial size,but a Delete will never shrink the size of the a table or its indexes.

About Dropping
Dropping a table removes the data and definition of the table. The indexes, constraints, triggers, and privileges on the table are also dropped. The action of dropping a table cannot be undone. The views, materialized views or other stored programs that reference the table are not dropped but they are marked as invalid.

SQL PL/SQL Question 16

Explain the difference between a FUNCTION, PROCEDURE and PACKAGE.

Procedures and functions are stored in compiled form in database.
Functions take zero or more parameters and return a value. Procedures take zero or more parameters and return no values.
Both functions and procedures can take or return zero or more values through their parameter lists.
Another difference between procedures and functions, other than the return value, is how they are called. Procedures are called as stand-alone executable statements:
my_procedure(parameter1,parameter2...);
Functions can be called anywhere in an valid expression :
e.g
1) IF (tell_salary(empno) < 500 ) THEN … 2) var1 := tell_salary(empno); 3) DECLARE var1 NUMBER DEFAULT tell_salary(empno); BEGIN …
Packages contain function , procedures and other data structures.
There are a number of differences between packaged and non-packaged PL/SQL programs. Package The data in package is persistent for the duration of the user’s session.The data in package thus exists across commits in the session.
If you grant execute privilege on a package, it is for all functions and procedures and data structures in the package specification. You cannot grant privileges on only one procedure or function within a package. You can overload procedures and functions within a package, declaring multiple programs with the same name. The correct program to be called is decided at runtime, based on the number or datatypes of the parameters.

SQL PL/SQL Question 17

Describe the use of %ROWTYPE and %TYPE in PL/SQL

%ROWTYPE associates a variable to an entire table row.
The %TYPE associates a variable with a single column type.

SQ PL/SQL Question 18

What are SQLCODE and SQLERRM and why are they important for PL/SQL developers?

SQLCODE returns the current database error number. These error numbers are all negative, except NO_DATA_FOUND, which returns +100.
SQLERRM returns the textual error message.. These are used in exception handling.

SQL PL/SQL Question 19

How can you find within a PL/SQL block, if a cursor is open?

By the Use of %ISOPEN cursor variable.

SQL PL/SQL Question 20

How do you debug output from PL/SQL?

By the use the DBMS_OUTPUT package.
By the use of SHOW ERROR command, but this only shows errors.
The package UTL_FILE can also be used.

SQL PL/SQL Question 21

What are the types of triggers?

Use Row and Statement Triggers

Use INSTEAD OF Triggers

SQL PL/SQL Question 22

Explain the usage of WHERE CURRENT OF clause in cursors ?

It refers to the latest row fetched from a cursor in an update and delete statement.

SQL PL/SQL Question 23

Name the tables where characteristics of Package, procedure and functions are stored ?

User_objects, User_Source and User_error.

SQL PL/SQL Question 24

What are two parts of package ?

They consist of package specification, which contains the function headers, procedure headers, and externally visible data structures. The package also contains a package body, which contains the declaration, executable, and exception handling sections of all the bundled procedures and functions.

SQL PL/SQL Question 25

What are two virtual tables available during database trigger execution ?

The table columns are referred as OLD.column_name and NEW.column_name.
For INSERT only TRIGGERS NEW.column_name values ARE only available.
For UPDATE only TRIGERS OLD.column_name NEW.column_name values ARE only available.
For DELETE only TRIGGERS OLD.column_name values ARE only available.v

SQL PL/SQL Question 26

What is Overloading of procedures ?

REPEATING OF SAME PROCEDURE NAME WITH DIFERENT PARAMETER LIST.

SQL PL/SQL Question 27

What are the return values of functions SQLCODE and SQLERRM ?

SQLCODE returns the latest code of the error that has occurred.
SQLERRM returns the relevant error message of the SQLCODE.

SQL PL/SQL Question 28

Is it possible to use Transaction control Statements such a ROLLBACK or COMMIT in Database Trigger ? Why ?

It is not possible.,because of the side effect to transactions. You can use them indirectly by calling procedures or functions .

SQL PL/SQL Question 29

What are the modes of parameters that can be passed to a procedure ?

IN, OUT, IN-OUT parameters.

Monday, September 14, 2015

Must Know PL/SQL Interview Questions - Latest PL SQl Interview Questions

1. What is PL/SQL ?
PL/SQL is a procedural language which has interactive SQL, as well as procedural programming language constructs like conditional branching and iteration.

2. Differentiate between % ROWTYPE and TYPE RECORD.
% ROWTYPE is used when a query returns an entire row of a table or view.
TYPE RECORD, on the other hand, is used when a query returns column of different tables or views.
Eg. TYPE r_emp is RECORD (sno smp.smpno%type,sname smp sname %type)
e_rec smp ROWTYPE
Cursor c1 is select smpno,dept from smp;
e_rec c1 %ROWTYPE

3. Explain uses of cursor.
Cursor is a named private area in SQL from which information can be accessed. They are required to process each row individually for queries which return multiple rows.

4. Show code of a cursor for loop.
Cursor declares %ROWTYPE as loop index implicitly. It then opens a cursor, gets rows of values from the active set in fields of the record and shuts when all records are processed.
Eg. FOR smp_rec IN C1 LOOP
totalsal=totalsal+smp_recsal;
ENDLOOP;

5. Explain the uses of database trigger.
A PL/SQL program unit associated with a particular database table is called a database trigger. It is used for :
1)Audit data modifications.
2)Log events transparently.
3)Enforce complex business rules.
4)Maintain replica tables
5)Derive column values
6)Implement Complex security authorizations

6. What are the two types of exceptions.
Error handling part of PL/SQL block is called Exception. They have two types : user_defined and predefined.

7. Show some predefined exceptions.
DUP_VAL_ON_INDEX
ZERO_DIVIDE
NO_DATA_FOUND
TOO_MANY_ROWS
CURSOR_ALREADY_OPEN
INVALID_NUMBER
INVALID_CURSOR
PROGRAM_ERROR
TIMEOUT _ON_RESOURCE
STORAGE_ERROR
LOGON_DENIED
VALUE_ERROR
etc.

8. Explain Raise_application_error.
It is a procedure of package DBMS_STANDARD that allows issuing of user_defined error messages from database trigger or stored sub-program.

9.Show how functions and procedures are called in a PL/SQL block.
Function is called as a part of an expression.
total:=calculate_sal(‘b644’)
Procedure is called as a statement in PL/SQL.
calculate_bonus(‘b644’);

10. Explain two virtual tables available at the time of database trigger execution.
Table columns are referred as THEN.column_name and NOW.column_name.
For INSERT related triggers, NOW.column_name values are available only.
For DELETE related triggers, THEN.column_name values are available only.
For UPDATE related triggers, both Table columns are available.

11. What are the rules to be applied to NULLs whilst doing comparisons?
1) NULL is never TRUE or FALSE
2) NULL cannot be equal or unequal to other values
3) If a value in an expression is NULL, then the expression itself evaluates to NULL except for concatenation operator (||)

12. How is a process of PL/SQL compiled?
Compilation process includes syntax check, bind and p-code generation processes.
Syntax checking checks the PL/SQL codes for compilation errors. When all errors are corrected, a storage address is assigned to the variables that hold data. It is called Binding. P-code is a list of instructions for the PL/SQL engine. P-code is stored in the database for named blocks and is used the next time it is executed.

13. Differentiate between Syntax and runtime errors.
A syntax error can be easily detected by a PL/SQL compiler. For eg, incorrect spelling.
A runtime error is handled with the help of exception-handling section in an PL/SQL block. For eg, SELECT INTO statement, which does not return any rows.

14. Explain Commit, Rollback and Savepoint.
For a COMMIT statement, the following is true:

Other users can see the data changes made by the transaction.
The locks acquired by the transaction are released.
The work done by the transaction becomes permanent.

A ROLLBACK statement gets issued when the transaction ends, and the following is true.

The work done in a transition is undone as if it was never issued.
All locks acquired by transaction are released.

It undoes all the work done by the user in a transaction. With SAVEPOINT, only part of transaction can be undone.

15. Define Implicit and Explicit Cursors.
A cursor is implicit by default. The user cannot control or process the information in this cursor.
If a query returns multiple rows of data, the program defines an explicit cursor. This allows the application to process each row sequentially as the cursor returns it.

16. Explain mutating table error.
It occurs when a trigger tries to update a row that it is currently using. It is fixed by using views or temporary tables, so database selects one and updates the other.

17. When is a declare statement required?
DECLARE statement is used by PL/SQL anonymous blocks such as with stand alone, non-stored procedures. If it is used, it must come first in a stand alone file.

18. How many triggers can be applied to a table?
A maximum of 12 triggers can be applied to one table.

19. What is the importance of SQLCODE and SQLERRM?
SQLCODE returns the value of the number of error for the last encountered error whereas SQLERRM returns the message for the last error.

20. If a cursor is open, how can we find in a PL/SQL Block?
the %ISOPEN cursor status variable can be used.

21. Show the two PL/SQL cursor exceptions.
Cursor_Already_Open
Invaid_cursor

22. What operators deal with NULL?
NVL converts NULL to another specified value.
var:=NVL(var2,’Hi’);
IS NULL and IS NOT NULL can be used to check specifically to see whether the value of a variable is NULL or not.

23. Does SQL*Plus also have a PL/SQL Engine?
No, SQL*Plus does not have a PL/SQL Engine embedded in it. Thus, all PL/SQL code is sent directly to database engine. It is much more efficient as each statement is not individually stripped off.

24. What packages are available to PL/SQL developers?
DBMS_ series of packages, such as, DBMS_PIPE, DBMS_DDL, DBMS_LOCK, DBMS_ALERT, DBMS_OUTPUT, DBMS_JOB, DBMS_UTILITY, DBMS_SQL, DBMS_TRANSACTION, UTL_FILE.

25. Explain 3 basic parts of a trigger.

A triggering statement or event.
A restriction
An action

26. What are character functions?
INITCAP, UPPER, SUBSTR, LOWER and LENGTH are all character functions. Group functions give results based on groups of rows, as opposed to individual rows. They are MAX, MIN, AVG, COUNT and SUM.

27. Explain TTITLE and BTITLE.
TTITLE and BTITLE commands that control report headers and footers.

28. Show the cursor attributes of PL/SQL.
%ISOPEN : Checks if the cursor is open or not
%ROWCOUNT : The number of rows that are updated, deleted or fetched.
%FOUND : Checks if the cursor has fetched any row. It is true if rows are fetched
%NOT FOUND : Checks if the cursor has fetched any row. It is True if rows are not fetched.

29. What is an Intersect?
Intersect is the product of two tables and it lists only matching rows.

30. What are sequences?
Sequences are used to generate sequence numbers without an overhead of locking. Its drawback is that the sequence number is lost if the transaction is rolled back.

31. How would you reference column values BEFORE and AFTER you have inserted and deleted triggers?

Using the keyword “new.column name”, the triggers can reference column values by new collection. By using the keyword “old.column name”, they can reference column vaues by old collection.

32. What are the uses of SYSDATE and USER keywords?
SYSDATE refers to the current server system date. It is a pseudo column. USER is also a pseudo column but refers to current user logged onto the session. They are used to monitor changes happening in the table.

33. How does ROWID help in running a query faster?
ROWID is the logical address of a row, it is not a physical column. It composes of data block number, file number and row number in the data block. Thus, I/O time gets minimized retrieving the row, and results in a faster query.

34. What are database links used for?
Database links are created in order to form communication between various databases, or different environments like test, development and production. The database links are read-only to access other information as well.

35. What does fetching a cursor do?
Fetching a cursor reads Result Set row by row.

36. What does closing a cursor do?
Closing a cursor clears the private SQL area as well as de-allocates memory

37. Explain the uses of Control File.
It is a binary file. It records the structure of the database. It includes locations of several log files, names and timestamps. They can be stored in different locations to help in retrieval of information if one file gets corrupted.

38. Explain Consistency
Consistency shows that data will not be reflected to other users until the data is commit, so that consistency is maintained.

39. Differ between Anonymous blocks and sub-programs.
Anonymous blocks are unnamed blocks that are not stored anywhere whilst sub-programs are compiled and stored in database. They are compiled at runtime.

40. Differ between DECODE and CASE.
DECODE and CASE statements are very similar, but CASE is extended version of DECODE. DECODE does not allow Decision making statements in its place.
select decode(totalsal=12000,’high’,10000,’medium’) as decode_tesr from smp where smpno in (10,12,14,16);
This statement returns an error.

CASE is directly used in PL/SQL, but DECODE is used in PL/SQL through SQL only.

41. Explain autonomous transaction.
An autonomous transaction is an independent transaction of the main or parent transaction. It is not nested if it is started by another transaction.
There are several situations to use autonomous transactions like event logging and auditing.

42. Differentiate between SGA and PGA.
SGA stands for System Global Area whereas PGA stands for Program or Process Global Area. PGA is only allocated 10% RAM size, but SGA is given 40% RAM size.

43. What is the location of Pre_defined_functions.
They are stored in the standard package called “Functions, Procedures and Packages”

44. Explain polymorphism in PL/SQL.
Polymorphism is a feature of OOP. It is the ability to create a variable, an object or function with multiple forms. PL/SQL supports Polymorphism in the form of program unit overloading inside a member function or package..Unambiguous logic must be avoided whilst overloading is being done.

45. What are the uses of MERGE?
MERGE is used to combine multiple DML statements into one.
Syntax : merge into tablename
using(query)
on(join condition)
when not matched then
[insert/update/delete] command
when matched then
[insert/update/delete] command

46. Can 2 queries be executed simultaneously in a Distributed Database System?
Yes, they can be executed simultaneously. One query is always independent of the second query in a distributed database system based on the 2 phase commit.

47. Explain Raise_application_error.
It is a procedure of the package DBMS_STANDARD that allow issuing a user_defined error messages from the database trigger or stored sub-program.

48. What is out parameter used for eventhough return statement can also be used in pl/sql?
Out parameters allows more than one value in the calling program. Out parameter is not recommended in functions. Procedures can be used instead of functions if multiple values are required. Thus, these procedures are used to execute Out parameters.

49. How would you convert date into Julian date format?
We can use the J format string :
SQL > select to_char(to_date(‘29-Mar-2013’,’dd-mon-yyyy’),’J’) as julian from dual;

JULIAN

50. Explain SPOOL
Spool command can print the output of sql statements in a file.
spool/tmp/sql_outtxt
select smp_name, smp_id from smp where dept=’accounts’;
spool off;

Wednesday, March 4, 2015

PL/SQL - Date & Time

PL/SQL provides two classes of date and time related data types:

Datetime data types
Interval data types

The Datetime data types are:

DATE
TIMESTAMP
TIMESTAMP WITH TIME ZONE
TIMESTAMP WITH LOCAL TIME ZONE

The Interval data types are:

INTERVAL YEAR TO MONTH
INTERVAL DAY TO SECOND

Field Values for Datetime and Interval Data Types

Both datetime and interval data types consist of fields. The values of these fields determine the value of the datatype. The following table lists the fields and their possible values for datetimes and intervals.

Field Name	Valid Datetime Values	Valid Interval Values
YEAR	-4712 to 9999 (excluding year 0)	Any nonzero integer
MONTH	01 to 12	0 to 11
DAY	01 to 31 (limited by the values of MONTH and YEAR, according to the rules of the calendar for the locale)	Any nonzero integer
HOUR	00 to 23	0 to 23
MINUTE	00 to 59	0 to 59
SECOND	00 to 59.9(n), where 9(n) is the precision of time fractional seconds The 9(n) portion is not applicable for DATE.	0 to 59.9(n), where 9(n) is the precision of interval fractional seconds
TIMEZONE_HOUR	-12 to 14 (range accommodates daylight savings time changes) Not applicable for DATE or TIMESTAMP.	Not applicable
TIMEZONE_MINUTE	00 to 59 Not applicable for DATE or TIMESTAMP.	Not applicable
TIMEZONE_REGION	Not applicable for DATE or TIMESTAMP.	Not applicable
TIMEZONE_ABBR	Not applicable for DATE or TIMESTAMP.	Not applicable

The Datetime Data Types and Functions

Following are the Datetime data types:

DATE - it stores date and time information in both character and number datatypes. It is made of information on century, year, month, date, hour, minute, and second. It is specified as:
TIMESTAMP - it is an extension of the DATE datatype. It stores the year, month, and day of the DATE datatype, along with hour, minute, and second values. It is useful for storing precise time values.
TIMESTAMP WITH TIME ZONE - it is a variant of TIMESTAMP that includes a time zone region name or a time zone offset in its value. The time zone offset is the difference (in hours and minutes) between local time and UTC. This datatype is useful for collecting and evaluating date information across geographic regions.
TIMESTAMP WITH LOCAL TIME ZONE - it is another variant of TIMESTAMP that includes a time zone offset in its value.

Following table provides the Datetime functions (where, x has datetime value):

S.N	Function Name & Description
1	ADD_MONTHS(x, y); Adds y months to x.
2	LAST_DAY(x); Returns the last day of the month.
3	MONTHS_BETWEEN(x, y); Returns the number of months between x and y.
4	NEXT_DAY(x, day); Returns the datetime of the next day after x.
5	NEW_TIME; Returns the time/day value from a time zone specified by the user.
6	ROUND(x [, unit]); Rounds x;
7	SYSDATE(); Returns the current datetime.
8	TRUNC(x [, unit]); Truncates x.

Timestamp functions (where, x has a timestamp value):

S.N	Function Name & Description
1	CURRENT_TIMESTAMP(); Returns a TIMESTAMP WITH TIME ZONE containing the current session time along with the session time zone.
2	EXTRACT({ YEAR \| MONTH \| DAY \| HOUR \| MINUTE \| SECOND } \| { TIMEZONE_HOUR \| TIMEZONE_MINUTE } \| { TIMEZONE_REGION \| } TIMEZONE_ABBR ) FROM x) Extracts and returns a year, month, day, hour, minute, second, or time zone from x;
3	FROM_TZ(x, time_zone); Converts the TIMESTAMP x and time zone specified by time_zone to a TIMESTAMP WITH TIMEZONE.
4	LOCALTIMESTAMP(); Returns a TIMESTAMP containing the local time in the session time zone.
5	SYSTIMESTAMP(); Returns a TIMESTAMP WITH TIME ZONE containing the current database time along with the database time zone.
6	SYS_EXTRACT_UTC(x); Converts the TIMESTAMP WITH TIMEZONE x to a TIMESTAMP containing the date and time in UTC.
7	TO_TIMESTAMP(x, [format]); Converts the string x to a TIMESTAMP.
8	TO_TIMESTAMP_TZ(x, [format]); Converts the string x to a TIMESTAMP WITH TIMEZONE.

Examples:

The following code snippets illustrate the use of the above functions:

SELECT SYSDATE FROM DUAL;

Output:

08/31/2012 5:25:34 PM

SELECT TO_CHAR(CURRENT_DATE, 'DD-MM-YYYY HH:MI:SS') FROM DUAL;

Output:

31-08-2012 05:26:14

SELECT ADD_MONTHS(SYSDATE, 5) FROM DUAL;

Output:

01/31/2013 5:26:31 PM

SELECT LOCALTIMESTAMP FROM DUAL;

Output:

8/31/2012 5:26:55.347000 PM

The Interval Data Types and Functions

Following are the Interval data types:

INTERVAL YEAR TO MONTH - it stores a period of time using the YEAR and MONTH datetime fields.
INTERVAL DAY TO SECOND - it stores a period of time in terms of days, hours, minutes, and seconds.

Interval functions:

S.N	Function Name & Description
1	NUMTODSINTERVAL(x, interval_unit); Converts the number x to an INTERVAL DAY TO SECOND.
2	NUMTOYMINTERVAL(x, interval_unit); Converts the number x to an INTERVAL YEAR TO MONTH.
3	TO_DSINTERVAL(x); Converts the string x to an INTERVAL DAY TO SECOND.
4	TO_YMINTERVAL(x); Converts the string x to an INTERVAL YEAR TO MONTH.

Monday, December 22, 2014

PL/SQL - Triggers

Triggers are stored programs, which are automatically executed or fired when some events occur. Triggers are, in fact, written to be executed in response to any of the following events:

A database manipulation (DML) statement (DELETE, INSERT, or UPDATE).
A database definition (DDL) statement (CREATE, ALTER, or DROP).
A database operation (SERVERERROR, LOGON, LOGOFF, STARTUP, or SHUTDOWN).

Triggers could be defined on the table, view, schema, or database with which the event is associated.

Benefits of Triggers

Triggers can be written for the following purposes:

Generating some derived column values automatically
Enforcing referential integrity
Event logging and storing information on table access
Auditing
Synchronous replication of tables
Imposing security authorizations
Preventing invalid transactions

Creating Triggers

The syntax for creating a trigger is:

CREATE [OR REPLACE ] TRIGGER trigger_name 
{BEFORE | AFTER | INSTEAD OF } 
{INSERT [OR] | UPDATE [OR] | DELETE} 
[OF col_name] 
ON table_name 
[REFERENCING OLD AS o NEW AS n] 
[FOR EACH ROW] 
WHEN (condition)  
DECLARE
   Declaration-statements
BEGIN 
   Executable-statements
EXCEPTION
   Exception-handling-statements
END;

Where,

CREATE [OR REPLACE] TRIGGER trigger_name: Creates or replaces an existing trigger with the trigger_name.
{BEFORE | AFTER | INSTEAD OF} : This specifies when the trigger would be executed. The INSTEAD OF clause is used for creating trigger on a view.
{INSERT [OR] | UPDATE [OR] | DELETE}: This specifies the DML operation.
[OF col_name]: This specifies the column name that would be updated.
[ON table_name]: This specifies the name of the table associated with the trigger.
[REFERENCING OLD AS o NEW AS n]: This allows you to refer new and old values for various DML statements, like INSERT, UPDATE, and DELETE.
[FOR EACH ROW]: This specifies a row level trigger, i.e., the trigger would be executed for each row being affected. Otherwise the trigger will execute just once when the SQL statement is executed, which is called a table level trigger.
WHEN (condition): This provides a condition for rows for which the trigger would fire. This clause is valid only for row level triggers.

Example:

To start with, we will be using the CUSTOMERS table we had created and used in the previous chapters:

Select * from customers;

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
+----+----------+-----+-----------+----------+

The following program creates a row level trigger for the customers table that would fire for INSERT or UPDATE or DELETE operations performed on the CUSTOMERS table. This trigger will display the salary difference between the old values and new values:

CREATE OR REPLACE TRIGGER display_salary_changes
BEFORE DELETE OR INSERT OR UPDATE ON customers
FOR EACH ROW
WHEN (NEW.ID > 0)
DECLARE
   sal_diff number;
BEGIN
   sal_diff := :NEW.salary  - :OLD.salary;
   dbms_output.put_line('Old salary: ' || :OLD.salary);
   dbms_output.put_line('New salary: ' || :NEW.salary);
   dbms_output.put_line('Salary difference: ' || sal_diff);
END;
/

When the above code is executed at SQL prompt, it produces the following result:

Trigger created.

Here following two points are important and should be noted carefully:

OLD and NEW references are not available for table level triggers, rather you can use them for record level triggers.
If you want to query the table in the same trigger, then you should use the AFTER keyword, because triggers can query the table or change it again only after the initial changes are applied and the table is back in a consistent state.
Above trigger has been written in such a way that it will fire before any DELETE or INSERT or UPDATE operation on the table, but you can write your trigger on a single or multiple operations, for example BEFORE DELETE, which will fire whenever a record will be deleted using DELETE operation on the table.

Triggering a Trigger

Let us perform some DML operations on the CUSTOMERS table. Here is one INSERT statement, which will create a new record in the table:

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (7, 'Kriti', 22, 'HP', 7500.00 );

When a record is created in CUSTOMERS table, above create trigger display_salary_changes will be fired and it will display the following result:

Old salary:
New salary: 7500
Salary difference:

Because this is a new record so old salary is not available and above result is coming as null. Now, let us perform one more DML operation on the CUSTOMERS table. Here is one UPDATE statement, which will update an existing record in the table:

UPDATE customers
SET salary = salary + 500
WHERE id = 2;

When a record is updated in CUSTOMERS table, above create trigger display_salary_changes will be fired and it will display the following result:

Old salary: 1500
New salary: 2000
Salary difference: 500