Friday, 5 June 2015

Oracle Apps Interview Question and Answer - SQL & PLSQL

1.   Advantages of stored functions and procedures
·         Applications can be modularized.
·         Easy maintenance.

-          Routines can be modified online without interfering other users.
-          One routine can be modified to effect multiple applications.
*   Improved data security and integrity.
-          Indirect access to database objects can be controlled from non-privileged users with security privileges.
*   Improved performance.
-          Reparsing for multiple users can be avoided by exploiting the shared SQL area.
-          PL/SQL parsing at run-time can be avoided by pursing at compile time.
-          Number of calls to the database can be reduced and network traffic decreased by bundling commands.
*   Improved code clarity.
-          The clarity of code increases by using appropriate identifier names to describe the
action of the routines which reduces the need for comments.

2.    Difference between a function and a procedure

                   Functions                                            Procedures

* Invoke as a part of an expression.                     Execute as a PL/SQL statement.
* Must contain a RETURN clause in the header.               Do not contain a RETURN
Clause in the header.
* Must return a single value.                                Can return none, one or many
values.
* Must contain at fast one RETURN                       Can contain a RETURN
   Statement.                                                Statement.
* Do not contain OUT and INOUT                         Can contain IN, Out and
             IN OUT parameters.                                              IN OUT Parameters.

3.    About Cursors
-          Oracle server uses some private work areas to execute SQL statements and to store processing information.
*   By using PL/SQL cursors these private SQL areas can be named and the stored
      information can be accessed.

Two Types:

* Implicit Cursors.
-          Implicit cursors are declared by PL/SQL implicitly for all DML and PL/SQL select statements, including queries that return only one row.
-          Oracle Server implicitly opens a cursor to process each SQL statement not associated with on explicitly declared cursor.
-          The most recent implicit cursor can be returned as the SQL cursor.

* Explicit Cursors
-          For queries that return more than one row, explicit cursors are declared and named by the programmers and manipulated through specific statements in the block’s executable actions.
-          Explicit cursors are used to individually process each row returned by a multiple-row SELECT statement.
-          The set of rows returned by a multiple – row query is called as active set.

Declare             Open            Fetch                     Empty?                  Close

 

Cursor Attributes:-

Attribute                    Type                     Description
% is open                    Boolean        Evaluates to TRUE if the cursor is open.
% not found                Boolean        Evaluates to TRUE if the most recent fetch
                                                     doesn’t return a row.
% found                      Boolean        Evaluate to TRUE if the most recent fetch
returns a row. Complement of % not found.
      % Row Count              Number        Evaluates the total number of rows returned so
far.   

Parameterized Cursors:-

-          Parameters can be passed to the cursor in a cursor for loop.
-          It allow to open and close an explicit cursor several times in a block, returning a different active set on each occasion for each execution, the previous cursor is closed and reopened with a new set of parameters.
-          Sizes should not be mentioned for the data types of parameters the parameters names are for references in the query expression of the cursor.

4.    Confined Mode:-
-          If it is on, child objects cannot be moved outside their enclosing parent objects.
-          If it is off child objects can be moved out sides their enclosing parent objects.

Flex Mode:-

-          If it is on, parent borders stretch when child objects are moved against them.
-          If it is off, parent borders remain fixed when child objects are moved against them.

5.    Parameters
-          A parameter is a variable whose value can be set at runtime (from the run time parameter of the command line).
-          User parameters are created by the user and system parameters are created by Report Builder.
-          System parameters cannot be renamed or deleted.


Bind Parameters (Variables)
-          Bind references (or Variables) are used to replace a single value in SQL or PL/SQL, such as a character string, number or date.
-          Bind references may be used to replace expressions in SELECT, WHERE, GROUP BY, ORDER BY, HAVING, CONNECT BY and START WITH clauses of queries.
-          Bind references cannot be referenced in FROM clauses.
-          Bind variables can be referenced by entering a colon (:) followed immediately by the column or parameter name.
-          If the parameter / column are not created before making a bind reference, report builder will create a parameter.

Lexical Parameters (Variables)
-          Lexical references are place holders for text that is embedded in a SELECT statement.
-          Lexical Variables can replace the clauses appearing after SLECT, FROM, WHERE, GROUP BY, ORDER BY, HAVING, CONNECT BY, and START WITH.

6.   What is % Row type
-          % Row types is used to declare a record based on a collection of columns in a database table or view.
-          The fields in the record take their names and data types from the columns of the table or view.
-          The record can also store an entire row of data fetched from a cursor or cursor variable.
-          % Row type should be prefixed with the database table.
Ex: Declare
     Emp_record   employee 5% row type.
Then emp_record will have a structure consisting of all the fields each representing a column in the employees table.

7.   What is a Ref Cursor?
-          Oracle server uses unnamed memory spaces to store data used in implicit cursors.
-          Ref cursors are used to define a cursor variable, which will point to that memory space and can be used like pointers in SQL ‘S’.

 

8.   About Exceptions
-          An exception is an identifier in PL/SQL that is raised during the execution of a black that terminates its main body of actions.
-          A block always terminates when PL/SQL raises an exception so that an exception handler should be specified to perform final actions.
* Exception can be raised in two ways exception is raised automatically.
  1. Ex:- when no rows are retrieved from the database in a SELECT statement, then error
  ORA-01403 occurs and the exception NO-DATA-FOUND is raised by PL/SQL.
  1. Ex:- Exception can be raised explicitly by issuing the RAISE statement with in the
  block.
-          The exception being raised may be either.

User-Defined or Pre Defined

Trapping an exception:-

-          If the exception is raised in executable section of the block, processing branches to the corresponding exception handler in the exception section of the block.
-          If PL/SQL successfully handles the exception, then the exception doesn’t propagate to the enclosing block or calling environment.
-          The PL/SQL block terminates successfully.

Propagating an exception:-

-          If the exception is raised in the executable section of the block and there is no corresponding exception handler, the PL/SQL block terminates with failure and the exception will be propagated to the calling environment.

Types of exceptions:-

A)   Pre-Defined Oracle Server Exceptions. -à Implicitly Raised.
B)   Non-Pre defined Oracle server exceptions. -à Implicitly Raised.
C)   User-defined exceptions -à Explicitly Raised.

Pre-Defined Oracle Server Exceptions:-

-          These are the error (20) that occurs most often in PL/SQL code.
-          These exceptions need not be declared and raised implicitly by Oracle Server, NO-DATA-FOUND, LOGIN_DENIED, ZERO_DIVIDE.

Non-Pre-Defined Oracle Server Exceptions:-

-          These are the other standard Oracle Server errors.
-          These exceptions need to be declared ion the declarative section and raised by Oracle server implicitly.

User Defined Exceptions:-

-          These are the conditions that the developer determines as abnormal.
-          These need to be declared and raised explicitly.

PRAGMA EXCEPTION_INIT

Statement is used to associate a declared exception with the standard Oracle Server error number.

Syntax: - PRAGMA EXCEPTION_INIT (exception, error number)

* SQLCODE, SQL ERRM are two functions which can be used to identify the associated error code or error message when exception occurs.
-          SQLCODE function returns the numeric value for the error code.
-          SQLERRM function returns the character data containing the message associated with the error number.
-          SQLCODE f SQLERRM cannot be used directly in SQL statements.

9.   What is Dynamic SQL?
-          Dynamic SQL is a SQL statement that contains variables that can change during runtime.
-          It is a SQL statement with place holders and is stored as a character string.
-          Dynamic SQL enables DDL, DCL or session control statements to be written and executed (by) from PL/SQL.
* Dynamic SQL can be written in two ways.
A)   DBMS_SQL. -à 8i
B)   Native Dynamic SQL.  -à 8i
-          Basically Dynamic SQL means creating the SQL statements dynamically at runtime by using variables.
Ex: - Dynamic SQL can be used to create a procedure that operates on a table whose name is not known until runtime or to execute DDL/DCL/SCS statements.
----à In Pl/SQL such statements cannot be executed statically.
--à EXECUTE IMMEDIATE Statement can perform dynamic single row queries.

Declare
D_str  varchar2 (200);
Val varchar2 (20);
Begin
D_str= insert into table1 values (;val);
Val= ‘ Bye’ Execute Immediate str using val; end;

10.What are Autonomous Transactions?
-          Autonomous transactions are the processes run independently of its parent.
-          By means of Autonomous Transaction, the current transaction can be temporarily suspended and another operation can be begun.
-          The basic idea behind this is to have some operation take place independently of the current transaction.

Ex:- to allow error messages written to table to be committed but to rollback everything
  else that has taken place prior to the error.
-          The autonomous or child transaction can commit or rollback as applicable with the execution of the parent transaction being resumed upon its completion.
-          The parent may then perform further operations of any operations performed with in the child transaction.
-          By using Autonomous Transactions, modular and reusable components can be developed more easily.
-          In fact Oracle already uses similar functionality internally, known as  recu   transactions to handle the updating of system resources.

Ex:- When one application selects ‘nextval’ from a non eached sequence, the value is in
  the database.
-          Thus a second application will always get the incremented application has committed or rolled back.
-          Autonomous Transaction should be defined in PL/SQL in the following manner.

PRAGMA AUTONOMOUS_TRANSACTION; 
-          Autonomous transaction also can be nested.
-          The parent transaction remains active while any statements specified in the declare section of the autonomous unit are executed.
-          As the code unit exits and control returns to the parent the main (parent) transaction is resumed and the transaction context is switched back to the parent.

11.What is Bulk binding of Bilk collect?

 

Bulkbind:-

-          The assignment of values to PL/SQL variables in SQL statements is called binding.
-          The binding of an entire collection at once is refilled to as bulk binding.
-          Bulk bind improves performance by minimizing the number of context switches between PL/SQL and SQL engines while they pass an entire collection of elements (varay, nested tables, index-by table or host array) as bind variables back and forth.
-          Prior to Oracle 81, the execution of every SQL statements required a switch between the Pl/SQL and SQL engines, where as bulk binds use only one context switch.
* Bulk binding includes the following
A)   Input collections; use the FORALL statement.
B)   Output collections, use the BULK COLLECT clause.

Input Collections:-

-          Input collections are data passed from Pl/SQL engine to the SQL engine to execute INSERT, UPDATE and DELETE statements.
Syntax:- FORALL index in lower_bound.. upper_bound sql_statement;

Output Collections:-

-          Output collections are the data passed from the SQL engine to the PL/SQL engine as a result of SELECT or FETCH statements.
-          The keyword BULK COLLECT can be used with SLECT INTO, FETCH INTO and RETURNING INTO clauses.
Syntax:- BULK COLLECT into collection_name, ……

12.What are Materialized Views and Snapshots?

Materialized View:-
-          A Materialized view is a replica of a target master from a single point in time.
-          In Oracle 7, it is terned as SNAPSHOT
-          Oracle 7.1.6 --à Uptable Snapshots
-          Oracle 7.3 -à Primary Key Snapshots
-          Oracle 8 -à Materialized view
-          Oracle 9 -à Multifier Materialized View.
-          Materialized views can be used both for creating summaries to be utilized in data warehouse environments.

* Replicating data in distributed environments.
Target Master -à The table(s) which is (are) referenced by the MVIEW query.
Best Table -à The tables are that is (are) created by MVIEW create statement and that stores data that satisfy the MVIEW query.
Syntax:- Create materialized view  <name>
                   Refresh fast
                   Start with sysdate
                   Next sysdate +1 as
                   Select *from <master table>;
-          Since this is a fast refreshed MVIEW the master table should have a log (Master log) to record the changes on it that can be created by running.

Create materialized view log on master_table;
-à this statement creats the following objects
-          a table called MLOG$_Master_table
-          an internal trigger on Master_table that populates the log table.
* Master Log tables (MLOG$) are used by fast refresh procedure.

Refreshing Materialized Views:-

-          Initially a materialized view contains the same data as in the master table.
-          After the MVIEW is created, changes can be made to the master table and possibly also to the MVIEW.
-          To keep a MVIEW data relatively current with the data in the master table, the MVIEW must be periodically refreshed.
* Refresh can be accomplished by one of the following procedures.
Dbms_mview.refresh (<mview list>, <Refresh types>)
Dbms _ refresh.refresh (<Refresh Groups>)

Refresh Types -à Complete Refresh, Fast Refresh, Force Refresh
* Complete Refresh is performed by deleting the rows from the snapshot and inserting the rows satisfying the MVIEW query.
* In Fast refresh only the rows updated since last refresh are pulled from the master table to insert into MVIEW.
* This requires a log table called as MVIEW Log to be created on the Master Table.
* Force refresh first tries to run a Fast refresh if possible.
* If fast refresh is not possible, it performs complete refresh.

Refresh Groups

-          These are used to refresh multiple snapshots in a transitionally consistent manner.
-          When a refresh group is refreshed all MVIEWS in that group are populated with data from a consistent point in time.
-          Refresh groups are managed by using the procedures in the package DBMS – REFRESH.
-          DBMS-REFRESH, MAKE of DBMS-REFRESH.ADD are used to create a refresh group and add new snap shots to an existing group.

Types of Materialized Views:- 

1.   Read-only materialized views
·         DML cannot be performed on the snapshots in this category.
2.   Up datable materialized views
·         These MVIEWS eliminates the restriction of DML’s on snapshots.
3     Sub query materialized views
·         These are the MVIEW’S that are created with sub queries in the WHERE clause of a MVIEW query.
4     Row id Vs Primary Key materialized views
·         MVIEW’S that use Row id for refresh are called Row id MVIEW’s (Oracle 7).
·         MVIEW’S  that use primary key for refresh are called primary key MVIE’S (Oracle 8).
* Fast refresh requires some association (mapping)
Between rows at snapshot and master tables.
5     Multifier materialized views (Oracle 9)
·         In this type MVIEW, its master table itself is a MVIEW.
·         This feature enables fast refresh of MVIEW’S that have MVIEW’S as their masters.
·         Many companies are structured on at least three levels
A)   International
B)   National
C)   Local
·         Many nodes at both the national and local levels are required
·         The best possible solution in such cases is to use multifier MVIEW’S.
6     Simple Vs Complex MVIEW’S.
·         MVIEW’S being simple or complex determines whether it can be fast refreshed or not.
·         A MVIEW is fast refreshable if it is simple.
·         A MVIEW is not fast refreshable if it is complex.
·         A MVIEW can be considered CONNECT BY, INTERSECT, MINUS or UNION or UNION ALL clauses in its detining query.
*   The following data types are not supported in MVIEW replication.
A)   LONG                          B) LONG RAW
C) BFILE                           D) UROWID
·         MVIEW’S are typically used in data ware house or decision support systems.

Snapshots

1.   Snapshots are mirror or replace of tables in a single point of time.
2.   A Snapshot is a local copy of a table data that originates from one or more remote master tables.
3.   To keep snapshots data current with the data of its master the Oracle server must periodically refresh the snapshot.

VIEWS

1.   Views are built  using the columns from one or more tables.
2.   The single table view can be updated, but the view with multiple tables cannot be updated.
- A snapshot contains a complete or partial copy of a target master table from a single point in time.
- A snapshot may be read only or up datable.

13. How duplicate rows are deleted?
3.   Duplicate rows are deleted by using ROWID
Syntax à delete from <Table>
              Where ROWID not in (Select max (ROWID) from  <Table>
                        Group by <Column_name>);

14. How do you call function and procedure in PL/SQL as well as in SQL prompt?
      isql*plus à EXECUTE < Function/Procedure name > ;
     (SQL prompt)

     PL/SQL à < Procedure Name / Function Name>;
    (from another procedure) 
    Development Tools à <Procedure name>;

15. Difference between IN and OUT parameters.
       Three types of parameters
          1. IN 
2. OUT        
3. IN OUT

IN parameter:
·         This parameter passes values from the calling environment into the procedure.
·         This is the default mode
·         A formal parameter of In mode cannot be assigned a value ( we IN parameter cannot be modified in the body of the procedure )
·         IN parameters can be assigned a default value in the parameter list.
·         IN parameters are passed by reference.
OUT parameters: 
·         OUT parameter must be assigned a value before returning to the calling environment.
·         OUT parameter passes a value from the procedure to the calling environment
·         OUT parameter can not be assigned a default value in the parameter list.
IN OUT parameter:
·         this type of parameter pass a value from the calling environment into the procedure and a possibly different value from the procedure back to calling environment using the same parameter.
·         IN OUT parameter cannot be assigned a default value. * By default OUT & IN OUT parameters are passed by value.
·         These can be passed by reference by using NOCOPY.

16. Triggers:
·         A trigger is a PL/SQL block or a PL/SQL procedure associated with a table view schema or the database.
·         The code in the trigger executes implicitly whenever a particular event occurs.

Two types of triggers:

Application trigger

·         fires whenever an event occurs with in a particular application

 

Database Trigger

·         Fires whenever a data event (Such as DML ) or system event (such as log on or shut down) occurs on a schema or database.
·         Executes implicitly when a data event. Such as DML on a table (insert, delete or Update), an INSTEAD OF trigger on a VIEW or DDL statements are issued no matter which user is connected or which application is used.
·         Also executes implicitly when some user or data base system actions occur.
·         Ex. When user logs on to the system.
·         When DBA shuts down the data base.
·         Date base triggers can be defined on tables and on views.
·          If a DML operations as issued on a view, the INSTEAD OF trigger defines what action takes place, if these actions include any DML operations on tables, then any triggers on the base tables are fired.
·         Data base triggers can be system triggers on a data base or a schema.
·         With a data base, triggers fire for each event for all users, with a schema, triggers fire for each event for the specific user.

Recursive trigger :
·         This is a trigger that contains a DML operation changing the very same table.

Cascading Trigger:
·         The action of one trigger cascades to another trigger, causing this second trigger to fire.
·         Oracle  server allows up to 32 triggers to cascade at any one time.
·         This number can be changed by changing the value of the OPEN  - CORSORS. Data bases initialization parameter. ( default value is 50 ).
·         * A triggering statement should contain

1 Trigger Timing               Before, After  (For Table)
Instead of     (For View)
·         Determines when the trigger needs to be fired in relation to the triggering event.
2 Triggering Event            Insert, Update, Delete
·         Determines which on the table or view causes the trigger to fire.
3 Trigger Type        Statements, Row
·         Determines how many times the trigger body executes
4 Table name                   Table, View
5 Trigger body à PL/SQL – block
·         Determines what actions the trigger should perform.

* INSTEAD of triggers are used to provide a transparent way to modifying views that
Can not be modified directly through SQL, DML statements because the view is not   
modifiable.
-à INSTEAD of triggers provide writing of Insert, Update and Delete statements against the view.
·         The INSTEAD if trigger works invisibly in the background performing the action coded in the trigger body directly on the underlying tables.
·         INSTEAD of trigger execute the trigger body instead of the triggering statement.

Statement Triggers

·         In this type of triggers, the trigger body executes once for the triggering event.
·         This is the default.
·         Statement trigger fires once, even if no rows are affected at all.

Row Trigger

·         In this type, the trigger body executes once for each row affected by the triggering event.
·          Row trigger is not executed if the triggering event affects no rows.

* A view cannot be modified by normal DML if the view query contains set operators, group functions, group by, connect By, start with clauses or joins.

Mutating Table

·         A Mutating table is a table that is currently being modified by an UPDATE, DELETE OR INSERT statement, or a table that might need to be updated by the effects of a declarative DELETE CASCADE referential integrity action.
·         A table is not considered mutating for statement triggers.
·         A mutating table cannot be changed because the resulting DML could change data that is in consistent state.

17.What is SQL Trace?  
·         SQL Trace is the main method for collecting SQL execution information in Oracle collecting a wide range of information and statistics that can be used to tune SQL operations.
  • The SQL – Trace facility can be enabled / disabled for an individual session or at the instance level.
  • If the initialization parameter SQL-TRACE is set to TRUE in the init.ora of an instance, then all sessions will be traced.
  • SQL-TRACE   can be set at the instance level by using the initialization parameter SQL-TRACE.
  • SQL-TRACE can also be enabled / disabled at the system/session level by using.
Alter system/session set SQL-TRACE = TRUE/FALSE.

Explain Plan

·         Explain plan command generates information that details the execution plan that will be used on a particular query.
·         A uses a pre created table (PLAN_TABLE) in the current schema to store information about the execution plan chosen by the optimizer.

à Creating the plan table
·         Plan table is created by using the script utl x plan, sql
(Oracle Home / RDBMS / admin / uti x plan.sql)
Unix à $ ORACLE_HOME / rdbms / admin
·         This script creates an output table, called PLAN-TABLE for holding the output of the explain command.

à Populating the PLAN TABLE
·         PLAN TABLE is populated using the explain plan.
SQL> Explain Plan for select * from emp where emp no = 1000;
·         This command inserts the execution plan of the SQL statement into the plan table.
·         A name tag can be added to explain information by using the set statement_id clause.

 

Displaying the Execution Plan

·         Once the table has been populated, the explain info needs to be retrieved and formatted.
·         Number of scripts are available to format the plan table data.

$ ORACLE_HOME / rdbms / admin / utlxpls. Sql – to format serial explain plans.
$ ORACLE_HOME/ rdbms/admin/utlxpil. Sql – to format parallel explain plans.

* AUTOTRACE
·         The AUTOTRACE facility in SQL* plus allows analysts to view the execution pan d some useful statistics for a SQL statement within a SQL*plus session.
·         AUTOTRACE needs to be initiated in the SQL*Plus session prior to executing the statement.

SET AUTOTRACE [OPTIONS] [EXPLAIN] [STATISTICS]
·         As with the explain plan command, to obtain an execution plan the PLAN-TABLE must be created in the user’s schema prior to Auto Tracing.

SQL> Set Auto trace trace only explain
SQL> Select * from dual;
·          
·         To enable viewing of STATISTICS data, the auto tracing user must have access to dynamic performance tables.
·         To achieve this, grant PLUS TRACE role to the user.

PLUS TRACE role is created by the plus trace. Sql script
$ ORACLE_HOME / sql plus admin
SYS user must run this script.
DBA can them grant the role to the users who wish to use the AUTOTRACE.

TK PROF 

·         Tk prof facility accepts as input a SQL Trace File and produces a formatted output file.
·         Tk Prof Filename_source filename_output EXPLAIN = [user name / password] sys = [yes/no] TABLE = [Table Name]

A) How do you add trace to a report?
4.   By using the package SRW.TRACE_ADD_OPTION
B) How do you execute a specified DDL in a report?
5.   BY using the package PW.DO_SQL
C) How do you generate message in reports?
-  By using the packages PW.MESSAGE (Reg Num,          );
      D) Explain BLOBS of CLOBS?

LOBà  A LOB is a data type that is used to store large, unstructured data such as text, graphic images, video, clippings etc.
* Four large object data types.

BLOB : Represents a binary large object
CLOB : Represents a character large object
NCLOB: Represents a multibyte character object.
BFILE: Represents a binary file store of in an os binary file outside the data base.
LOB’S  à Internal LOBS (CLOB, BLOB, NCLOB)
                   external Files (BFILE)
·         Depending on the storage aspects and their interpretation by Oracle server.
* LONG_TO_LOB API is used to migrate LONG columns to LOB columns.

 

LOB’S

          LOB Locator

·         A table can have multiple LOB columns
·         The maximum size of a LOB can be 4 GB
·         LOB’S return the locator
·         LOB’S store a locator in the table end data in a different segment unless the data is less than 4000 bytes.

LOBà LOB value (real data)
          LOB locator (pointer to the location of the LOB value)                 
·         A LOB column doesn’t contain the data and it contains the located of the LOB value.
·         When a table is created with LOB column, the default storage is ENABLE STORAGE IN ROW.
·         If DISABLE storage in Row option is used the LOB value is not stored in the ROW even if the size is less then 4000 bytes.

Internal LOB’S

·         Stored inside the Oracle server.
·         BLOB, NCLOB, CLOB.

BFILE

·         BFILE’S are external LOB’S.
·         These are stored in OS files out side the database table spaces.
·         The data type is BFILE.
·         BFILE data file stores a locator to the physical file.
·         BFILE can be GIF, JPEG, MPEG, text or other formats.

*DBMS_LO.READ of DBMS_lob.WRITE are used to manipulate LOBS.

  18. What is sub query and correlated sub query?

Sub Query :-
·         A Sub Query is a SELECT statement that is embedded in a clause of other SQL statements called the parent statement.
·         Sub query (Inner Query) returns a value that is used by the outer query.
·         Scalar sub query is a sub query that returns exactly one column value from one row.
Correlated Sub Query
·         Correlated sub query are used for row – by –row  processing.
·         Each sub query is executed once for every row of the outer query.
·         A correlated sub query is one way of reading (data) every row in a table and comparing values in each row against related data.
·         Oracle server performs correlated sub query when the sub query references a column from a table in the parent query. 
·         The inner query is driven by the outer query in correlated sub queries.

19.   What is Decode function?
  • Decode function decodes an expression in a way similar to the IF_THEN_ELSE logic used in various languages.
  • Decode function decodes expression after comparing it to each search condition.
  • If the expression is the same as search, result is returned.
  • If the default value is committed, a null value is returned where a search value does not match any of the result values.
  • DECODE function facilitates conditional inquiries by doing the work of a CASE or IF_THEN_ELSE statement.
DECODE (column, expression, search1, result1, search2, result2, …..); 
Ex:- Select last_name, job, alary,
          DECODE (      ‘IT_PROG’, 1.10*SALARY,
                             ‘ST_CLERK’, 1.15*SALARY,
                             ‘ST_REP’, 1.20*SALARY,
                             SALARY) REVISED_SALARY from employees;

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