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Exception Handling

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Exception Handling - Objectives

At the end of this session, the participants will be able to:

Understand the concept of Exceptions

Comprehend the difference between system and user-defined exceptions

Write PL/SQL blocks to handle system and user-defined exceptions

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Exception Handling - Topics

What are Exceptions?

System defined exceptions

User defined exceptions

Exception Propagation

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Exception Handling in PL/SQL

An exception is an identifier in PL/SQL, raised during the execution of a block that

terminates its main body of actions

How an exception is raised?1. An exception is raised implicitly if an Oracle error

occurs e. g. When no rows are retrieved bySELECTstatement then PL/SQL raises theexception NO_DATA_FOUND

2. We raise an exception explicitly by issuing the RAISEstatement within the block

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Functions for Trapping Error Code and Message

SQLCODEReturns the error number

SQLERRM

Returns the error message associated with a given error number

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Example

DECLAREv_code NUMBER;

v_errm varchar2(255);

BEGIN

. . .

. . .

EXCEPTION. . .

. . .

WHEN OTHERS THEN

v_code : = SQLCODE;

v_errm : = SQLERRM(SQLCODE,1,80);

INSERT INTO errors VALUES(v_code, v_errm);

END;

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Types of Exceptions

- Predefined Oracle Server Exceptions- Non-predefined Oracle Server Exceptions

- User-defined ExceptionsException Description Direction for Handling

Predefined Oracle

Server

Approximately 20 errors that

occur most often in PL/SQL

code

Do not declare and

allow the oracle server

to raise them implicitly

Non-predefined

Oracle Server

Any other standard Oracle

Server error

Declare within the

declarative section and

allow the oracle serverto raise them implicitly

User-defined A condition that the

developer determines is

abnormal

Declare within the

declarative section and

raise explicitly

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Pre-defined Exceptions

- Oracle has around 20 named Pre-definedexceptions

- Trapped within the Exception handling block

Pre-defined Exceptions

- NO_DATA_FOUND

- TOO_MANY_ROWS

- INVALID_CURSOR- ZERO_DIVIDE

- VALUE_ERROR

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Pre-define Exception

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Example

DECLARE

v_emp_rec emp%ROWTYPE;

BEGIN

SELECT * INTO v_emp_rec FROM emp WHERE empno=1001;

DBMS_OUTPUT.PUT_LINE(Empno : || v_emp_rec.empno);

DBMS_OUTPUT.PUT_LINE(Emp Name : || v_emp_rec.ename);

DBMS_OUTPUT.PUT_LINE(Emp Salary : || v_emp_rec.sal);

DBMS_OUTPUT.PUT_LINE(Emp Deptno : || v_emp_rec.deptno);EXCEPTION

WHEN NO_DATA_FOUNDTHEN

DBMS_OUTPUT.PUT_LINE(Employee Does Not Exist);

END;

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Trapping Non-predefined Oracle Server errors

- Trapped by declaring it first or by using OTHERShandler

- PRAGMAEXCEPTION_INITtells the compiler toassociate an exception name with an oracle error

number- PRAGMAkeyword that signifies that the statementis a compiler directive, which is not processedwhen the PL/SQL block is executed

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Example

DECLARE

e_emp_exist EXCEPTION;

--Naming non-predefined oracle errorPRAGMA EXCEPTION_INIT(e_emp_exist , -2292);

v_deptno dept.deptno%TYPE := &p_dno;

BEGIN

DELETE FROM deptWHERE deptno = v_deptno;

COMMIT;

EXCEPTION

WHEN e_emp_exist THEN

DBMS_OUTPUT.PUT_LINE(Can not delete dept Employees exist);

END;

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Oracle Server Errors

ORA-00000:normal, successful completion

Cause: Normal exit.Action: None.

ORA-00001:unique constraint (string.string) violated

Cause: An UPDATE or INSERT statement attempted to insert a duplicatekey. For Trusted Oracle configured in DBMS MAC mode, you may see

this message if a duplicate entry exists at a different level.Action: Either remove the unique restriction or do not insert the key.

ORA-00017:session requested to set trace event

Cause: The current session was requested to set a trace event by anothersession.

Action: This is used internally; no action is required. ORA-00018:maximum number of sessions exceeded

Cause: All session state objects are in use.

Action: Increase the value of the SESSIONS initialization parameter.

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ORA-00019:maximum number of session licenses exceeded

Cause: All licenses are in use.Action: Increase the value of the LICENSE MAX SESSIONS initialization

parameter.

ORA-02286:no options specified for ALTER SEQUENCE

Cause: Self-evident.

Action: The statement is meaningless without any options. ORA-02287:sequence number not allowed here

Cause: The specified sequence number (CURRVAL or NEXTVAL) isinappropriate here in the statement.

Action: Remove the sequence number.

ORA-02288:invalid OPEN mode

Cause: A token other than RESETLOGS appears following ALTERDATABASE OPEN.

Action: Either nothing or RESETLOGS should be placed following OPEN.

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ORA-02289: sequence does not exist

Cause: The specified sequence does not exist, or the user does not have therequired privilege to perform this operation.

Action: Make sure the sequence name is correct, and that you have the right to

perform the desired operation on this sequence.

ORA-02290:check constraint (string.string) violated

Cause: The values being inserted do not satisfy the named check constraint.

Action: do not insert values that violate the constraint.

ORA-02291:integrity constraint (string.string) violated - parent key not found

Cause: A foreign key value has no matching primary key value.

Action: Delete the foreign key or add a matching primary key.

ORA-02292:integrity constraint (string.string) violated - child record found

Cause: attempted to delete a parent key value that had a foreign key dependency.

Action: delete dependencies first then parent or disable constraint.

ORA-02293:cannot validate (string.string) - check constraint violated

Cause: an alter table operation tried to validate a check constraint to a populated

table that had nocomplying values.

Action: Obvious

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User-defined Exceptions

- Declared in the Declare Section with type asEXCEPTION

- Raised Explicitly by using RAISE keyword

- Handled in the Exception Section- Local to a PL/SQL block

DECLARE RAISE REFERENCE

Declarative Section Executable Section Exception Handling Section

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Example

DECLARE

-- Declaring user defined exceptione_invalid_product EXCEPTION;

BEGIN

UPDATE product

SET descrip = &prod_desc

WHERE prod_id = &prod_no;

IF SQL%NOTFOUND THEN

--Raising user defined exceptionRAISE e_invalid_product

END IF;

COMMIT;

EXCEPTION

--Handling user defined exceptionWHEN e_invalid_product THEN

DBMS_OUTPUT.PUT_LINE(Invalid Product Number);

END;

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RAISE_APPLICATION_ERROR

To display your own error messages one can use the built-inRAISE_APPLICATION_ERROR.

They display the error message in the same way as Oracle errors.

You should use a negative number between20000 to20999 for theerrornumber

The error message should not exceed 512 characters.

RAISE_APPLICATION_ERROR (error_number, error_message, { TRUE| FALSE });

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Example

BEGIN

DBMS_OUTPUT.PUT_LINE(Enter Your Name :: );str:=&NAME;

IF LENGTH(str) < 2 THEN

RAISE_APPLICATION_ERROR(-20888, What a Funny Name You have);

ELSE

RAISE_APPLICATION_ERROR(-20222, What a Lovely Name);

END IF;

END;

Enter value for name :LARRY

Old 5: str:=&NAME;

New 5: str:=LARRY;

ERROR at Line 1:

ORA-20222: What a Lovely Name

ORA-06512: at line 9

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Exceptions Raised in the Executable Section

PL/SQL uses the following rule to determine which exception handler toinvoke:

1. Current block has a handler for the exception,execute it and complete the block successfully.

Control then passes to the enclosing block.2. No handler for current exception, propagate the

exception by raising it in the enclosing block.Step 1 is executed for the enclosing block. Ifthere is no enclosing block, the exception will bepropagated out to the calling environment, suchas SQL* Plus.

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Exceptions Raised in the Executable Section: Example 1

DECLARE

A EXCEPTION;

BEGIN

BEGIN

RAISE A;EXCEPTION

WHEN A THEN

. . .

END;END;

Exception A is

raised in the

inner block

A is handledin the inner

block

Control

resumes

here

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Exceptions Raised in the Executable Section: Example 2

DECLARE

A EXCEPTION;B EXCEPTION;

BEGIN

BEGIN

RAISE B;

EXCEPTIONWHEN A THEN

. . .

END;

EXCEPTION

WHEN B THEN. . .

END;

Exception B

is raised in

the inner

block

No handler for

exception B in

the inner block

Exception B is propagated

to the enclosing block and

handle there

Control then passes out of

enclosing block, which completes

successfully

S

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Exceptions Raised in Declarative Section:Example 1

DECLARE

v_number NUMBER(3):=ABC;BEGIN

. . .

EXCEPTION

WHEN OTHERS THEN

. . .

END;

Illegal assignment

raises VALUE_ERROR

Even though there is a

WHEN OTHERS handler, itis not executed

The block completes

unsuccessfully with theVALUE_ERROR exception

E ti R i d i D l ti S ti

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Exceptions Raised in Declarative Section:Example 2

BEGIN

DECLARE

v_number NUMBER(3):=ABC;

BEGIN

. . .

EXCEPTION

WHEN OTHERS THEN

. . .

END;

EXCEPTION

WHEN OTHERS THEN. . .

END;

Illegal assignment

raises VALUE_ERROR

Even though there is an

OTHERS handler for the

inner block, it is not

executed

The exception handler

in the outer block

handles the exception

Control then passes

out of enclosing block

which completes

successfully

E ti R i d i E ti S ti

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Exceptions Raised in Exception Section:Example 1

DECLARE

A EXCEPTION;

B EXCEPTION;

BEGINRAISE A;

EXCEPTION

WHEN A THEN

RAISE B;

WHEN B THEN

. . .

END;

Exception A is

raised

Exception A is

handled and B is

raised in As handler

Even though there is a

handler for B here, it is

not executed. The exception

is propagated out of theblock

The block completes

unsuccessfully with

unhandled exception B

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Exception Handling

The biggest advantage of exception handling is it improves readability andreliability of the code.

Errors from many statements of code can be handles with a singlehandler.

Instead of checking for an error at every point we can just add anexception handler and if any exception is raised it is handled by that.

For checking errors at a specific spot it is always better to have thosestatements in a separate beginend block.

P i t T P d

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Points To Ponder:

An Exception cannot be declared twice in the same block. Exceptions declared in a block are considered as local to that block and

global to its sub-blocks.

An enclosing block cannot access Exceptions declared in its sub-block.Where as it possible for a sub-block to refer its enclosing Exceptions.

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Summary

We have discussed,

Terms : Exceptions and Exception handling

Predefined exceptions

User defined exceptions


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QuestionsThank You

004 exceptions 10g

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