Exception Handling

in PL/SQL

POINTS TO DISCUSS

• What is Exception Handling

• Structure of Exception Handling Section

• Types of Exceptions

What is Exception Handling

•Error condition in PL/SQL is called exception.

•A part of Executable section•Between Begin… and end;• Using Exception Handling we can test

the code and avoid it from exiting abruptly.

•When an exception occurs a message which explains its cause is received.

Exception consists of 3 parts

•Type of Exception

•An Error Code

•A message By handling exceptions we ensure a PL/SQL block does not exit abruptly.

Syntax for exception DECLARE

Exception Declaration BEGIN

Executable statements…EXCEPTION

WHEN ex_name1 THEN -Error handling statements WHEN ex_name2 THEN -Error handling statements WHEN Others THEN -Error handling statements

END;

When an exception is raised, Oracle searches for an appropriate exception handler in the exception section.

Only one exception can be raised in a Block and the control does not return to the Execution Section after the error is handled.

Types of Exceptions

•Pre defined Exceptions

•Unnamed System Exceptions

•User-defined Exceptions

Named System Exceptions (Predefined)

• Automatically raised by Oracle when a program violates a RDBMS rule

• System exceptions that are raised frequently are pre-defined and given a name in Oracle

• Not Declared explicitly• Raised implicitly when a predefined Oracle

error occurs• Caught by referencing the standard name

within an exception-handling routine

Some Named (Predefined) ExceptionsException Name Reason Error Number

CURSOR_ALREADY_OPEN When you open a cursor that is already open.

ORA-06511

INVALID_CURSOR When you perform an invalid operation on a cursor like closing a cursor, fetch data from a cursor that is not opened.

ORA-01001

NO_DATA_FOUND When a SELECT...INTO clause does not return any row from a table.

ORA-01403

TOO_MANY_ROWS When you SELECT or fetch more than one row into a record or variable.

ORA-01422

ZERO_DIVIDE When you attempt to divide a number by zero.

ORA-01476

Predefined Exception Exampledeclarev_empno emp.empno%type;v_ename emp.ename%type;v_sal emp.sal%type;a number;begina:=&a;select empno,ename,sal into v_empno,v_ename,v_sal from emp where empno=&v_empno;v_sal:=v_sal+v_sal/a;dbms_output.put_line('Employee number is '||v_empno || ' and name is '||v_ename);dbms_output.put_line('New salary is '||v_sal);exceptionwhen no_data_found thendbms_output.put_line('NO SUCH RECORD!!!');when zero_divide thendbms_output.put_line('Salary cannot be divided by zero');when others thendbms_output.put_line('Some error.GOD knows what???');end;/

Unnamed System Exceptions• System exception for which oracle does not

provide a name is known as unnamed system exception.

• These exceptions do not occur frequently.• These Exceptions have a code and an associated

message. • There are two ways to handle unnamed system

exceptions: 1. By using the WHEN OTHERS exception handler, or 2. By associating the exception code to a name and using it as a named exception.

Unnamed exceptions

•We can assign a name to unnamed system exceptions using Pragma EXCEPTION_INIT.

•EXCEPTION_INIT will associate a predefined Oracle error number to a programmer_defined exception name.

Syntax for unnamed system exception using EXCEPTION_INIT

DECLARE exception_name EXCEPTION; PRAGMA EXCEPTION_INIT (exception_name, Err_code); BEGIN Execution sectionEXCEPTION WHEN exception_name THEN handle the exceptionEND;

ExampleDECLAREdup_pr_key exception;pragma exception_init(dup_pr_key,-1);BEGINinsert into emp(empno,ename) values(1111,'ABCD');dbms_output.put_line('One record successfully inserted');insert into emp(empno,ename) values(1111,'EFGH');dbms_output.put_line('One more record successfully inserted');EXCEPTIONwhen dup_pr_key thendbms_output.put_line('How come more employees with the same employee number !!!');END;/

User-defined Exceptions

• They should be explicitly declared in the declaration section.

• They should be explicitly raised in the Execution Section.

• They should be handled by referencing the user-defined exception name in the exception section.

User defined Exception-Exampledeclarev_empno emp.empno%type;v_ename emp.ename%type;v_sal emp.sal%type;a number;lo_sal exception;begina:=&a;select empno,ename,sal into v_empno,v_ename,v_sal from emp where empno=&v_empno;v_sal:=v_sal+v_sal/a;if v_Sal<2000 thenraise lo_sal;end if;dbms_output.put_line('Employee number is '||v_empno || ' and name is '||v_ename);dbms_output.put_line('New salary is '||v_sal);exceptionwhen lo_sal then

dbms_output.put_line('In these times of Inflation INCREASE THE SALARY !!!');when others then

dbms_output.put_line('Some error.GOD knows what???');end;

RAISE_APPLICATION_ERROR ( )

• a built-in procedure in oracle used to display the user-defined error messages along with the error number

• range of error number between -20000 and -20999.

• Whenever a message is displayed using RAISE_APPLICATION_ERROR, all previous transactions which are not committed within the PL/SQL Block are rolled back automatically (i.e. change due to INSERT, UPDATE, or DELETE statements).

• raises an exception but does not handle it.

WHY RAISE_APPLICATION_ERROR ?

•to create a unique id for an user-defined exception.

• to make the user-defined exception look like an Oracle error.

Syntax

RAISE_APPLICATION_ERROR (error_number, error_message);

•The Error number must be between -20000 and -20999

•The Error message is the message you want to display when the error occurs.

Steps for RAISE_APPLICATION_ERROR procedure:

•Declare a user-defined exception in the declaration section.

•Raise the user-defined exception based on a specific business rule in the execution section.

•Finally, catch the exception and link the exception to a user-defined error number in RAISE_APPLICATION_ERROR.

RAISE_APPLICATION_ERROR -ExampleDECLAREv_empno emp.empno%type;v_ename emp.ename%type;v_sal emp.sal%type;a number;lo_sal exception;BEGINa:=&a;select empno,ename,sal into v_empno,v_ename,v_sal from emp

where empno=&v_empno;v_sal:=v_sal+v_sal/a;if v_Sal<2000 thenraise lo_sal;end if;dbms_output.put_line('Employee number is '||v_empno || ' and name is '||v_ename);dbms_output.put_line('New salary is '||v_sal);

RAISE_APPLICATION_ERROR –Example(contd.)EXCEPTIONwhen lo_sal thenRAISE_APPLICATION_ERROR(-20001,'In these times of Inflation INCREASE THE SALARY !!!');when others thendbms_output.put_line('Some error.GOD knows what???');END;

THANKS