Chapter Eighteen Exception Handling

Chapter 18: Exception Handling 1

Chapter EighteenException Handling

Objective:– Define exceptions

– List types of exception handlers

– Trap errors

– Exception messages & customization

– Blocks & exception propagation


Introduction

DECLAREPE_ratio NUMBER(3,1);

BEGINSELECT Price/earningINTO PE_ratioFROM StockWHERE Symbol = ‘BBY’;INSERT INTO table1(Symbol, ratio)VALUES (‘BBY’, PE_ratio);COMMIT;

EXCEPTIONWHEN ZERO_DIVIDE THEN

INSERT INTO table1(Symbol, ratio)VALUES (‘BBY’, NULL);COMMIT;

WHEN OTHERS THENROLLBACK;

END;


Adapting an Exception Handling Strategy

• How to log an error?• How to report an error to users?• When to log an error?• When to report an error to users?• Who should receive the error log & how to make

correction?• How to handle the transaction after an error occurred?• Where should I declare my error handling:

– For each block– Only for top level


Definition Of Exception Handling:

• “Methods of a program that react and deal with runtime errors”.

• ORA-1000 Unique constraint violated

• ORA-06502 PL/SQL: numeric or value error

• ORA-01403 No data is retrieved from SELECT statement (NO_DATA_FOUND)


Exception Handling

• Facts:– Exception is an identifier in PL/SQL

– Exception mechanism of PL/SQL is the same as ADA

– It is similar to JAVA: but unlike JAVA, exception is not an object

– Exceptions are designed for run-time error handling -not compile time.

– Unlike C, Oracle system has error handling methods


Exception Handling

How do we handle exceptions:

• Trap the exception

• Propagate it to the calling environment


Handling Exceptions

Trap the Exception

DECLARE

…..

BEGIN

….

EXCEPTION

…..

END

Propagate the ExceptionDECLARE

…..

BEGIN

….

EXCEPTION

…

END

Exception propagates to calling environment


Exception Handling

Trap the exception:DECLARE….BEGIN…. <Exception is raised>EXCEPTION…. <Exception is trapped>END

If exception is not handled successfully, block terminates with failure and the exception propagates to the calling block


Exception Types

1. Predefined Oracle Server

2. Non-predefined Oracle Server

3. User-defined


Exception Types:

1. Predefined Oracle Server errors:– No declaration is needed– Oracle Server raises them implicitly

ORA-01403 NO_DATA_FOUNDORA-01422 TOO_MANY_ROWSORA-01476 ZERO_DIVIDEORA-06500 STORAGE_ERRORORA-06530 ACCESS_INTO_NULLORA-06592 CASE_NOT_FOUNDORA-00001 DUP_VAL_ON_INDEX


Exception Types:

2. Non_Predefined Oracle Server errors– Other Standard Oracle Server errors– Declared within the declarative section– Oracle Server raises them implicitly


Exception Types:

3. User_defined– User determines an abnormal condition– Declared in declaration section– It is raised explicitly

DECLAREe_TooManyNumbers EXCEPTION;


Raising Exceptions:

• When an error occurs, an exception is raised

• User_defined exceptions are raised explicitly via RAISE command

• Other exceptions are raised by EXCEPTION_INIT pragma


Raising USER_DEFINED Exceptions

• Example

DECLAREe_TooManyNumbers EXCEPTION;V_NoStudent NUMBER(4);V_MaxStudent NUMBER(4);

BEGINSELECT Current_Students, Max_StudentsINTO V_NoStudent, V_MaxStudentFROM classesWHERE C_Num=641 AND Dept=‘COSC’;IF V_NoStudent > V_MaxStudent THEN

RAISE e_TooManyNumbers;END IF;

END;


Ways an Exception may be Raised

• Oracle raises an exception when it detects an error

• User may raise an exception with RAISE

• User may raise an exception with RAISE_APPLICATION_ERROR built in procedure


Type of RAISE Statement

• RAISE exception_name;

• RAISE Package_name.exception;

• RAISE;


Example

DECLAREinvalid_id EXCEPTION;id_value VARCHAR2;

BEGINid_value := id_for(‘Mike’);IF id_value BETWEEN 1111 AND 2222 THEN

….ELSE

RAISE invalid_id;END IF;

END;

• RAISE ZERO_DIVIDE; -- Predefined Oracle exception


Re-raise Exceptions

BEGINBEGIN

IF X=-10 THENRAISE X_IS_LOW;

END IF;EXCEPTION

WHEN X_IS_LOW THEN DBMS_OUTPUT.PUT_LINE (X || ’is low’);

RAISE;END…

EXCEPTIONWHEN X_IS_LOW THEN

--Handle Error HereEND;


Example

• RAISE;• Use this form if you want to re-raise the same exception

from within an exception handler

EXCEPTIONWHEN My_error THEN

DBMS_OUTPUT.PUT_LINE(‘Error description’);

--pass on the exception to the enclosing blockRAISE;

END;


RAISE_APPLICATION_ERROR(Customize The Error Messages)

SyntaxRAISE_APPLICATION_ERROR(err_No, Message)[, TRUE | FALSE];

• It lets you issue a non standard user defined error message from stored subprograms• Called only from an executing stored subprogram

…..BEGIN

SELECT count(*)INTO xFROM USERTABLE;IF x<100 THEN

RAISE_APPLICATION_ERROR(-20202, ‘Expect at least 100 rows’);ELSE

….END;


Example

BEGINDELETE FROM deptWHERE deptNo = V_deptNo;IF SQL%NOTFOUND THEN -- in execution part

RAISE_APPLICATION_ERROR(-20002, ‘Error in deleting depart no.’);END IF;……


Raising Predefined Exceptions

• Example

BEGIN

INSERT INTO Students (id, name)

VALUES (111, ‘Mark’);

INSERT INTO Students (id, name)

VALUES (111, ‘Mary’);

DUP_VAL_ON_INDEX


1-Trapping Exceptions

Syntax

EXCEPTIONWHEN exception1 THEN

statement1;statement2;…

[WHEN exception2 THENstatement1;statement2;

…][WHEN OTHERS THEN

statement1;statement2;….]


Trapping Exceptions Guidelines

• EXCEPTION Keyword starts exception handling section

• Several exception handlers are allowed• Only one handler is processed before leaving

the block• WHEN OTHERS is the Last clause:• Exceptions can not appear in assignment

statement or SQL statement


Several Places can Raise the Same Exception:

DECLAREe_TooManyNumbers EXCEPTION;V_NoStudent NUMBER(4);V_MaxStudent NUMBER(4);

BEGINSELECT Current_Students, Max_StudentsINTO V_NoStudent, V_MaxStudentFROM classesWHERE C_Num=641 AND Dept=‘COSC’;IF V_Student > V_MaxStudent THEN

RAISE e_TooManyNumbers;END IF;

Continued


Example

EXCEPTIONWHEN e_TooManyNumbers THEN

INSERT INTO log_file (info)

VALUES (‘COSC 641 has:‘ || V_NoStudent || ‘Max No is:’ || V_MaxStudent);

END;


Two or More Exceptions Executing the Same Sequence of StatementsIF …

RAISE a;IF …

RAISE b;…

EXCEPTIONWHEN a or b or c THEN

…


Example

EXCEPTION

WHEN NO_DATA_FOUND OR TOO_MANY_ROWS THEN


VALUES (‘ ‘);

WHEN OTHERS THEN


VALUES (‘ ‘);

END;


2 - Trapping Non_Predefined Oracle Server Errors

1. Declare the exception first

2. Code the PRAGMA EXCEPTION_INIT

3. Handle the raised exception


Non-Predefined Error Syntax

exceptionName EXCEPTION;

…

PRAGMA EXCEPTION_INIT(exceptionName, error_No);


Example

DECLAREe_Faculty_Remaining EXCEPTION;PRAGMA EXCEPTION_INIT (e_Faculty_Remaining, -6501);V_deptNo dept.deptNo%TYPE := &p_deptno;

BEGINDELETE FROM deptWHERE deptNo = V_deptNo;COMMIT;

EXCEPTIONWHEN e_faculty_Remaining THEN

DBMS_OUTPUT.PUT_LINE(‘can not remove dept, there are faculty in the dept’);

END;


3 - Trapping User_defined Exceptions

1. Declare the exception

2. Explicitly raise the exception by using RAISE statement

3. Handle the raised exception


User defined Error Syntax

exceptionName EXCEPTION:

…

RAISE exceptionName;


Example

DECLAREe_Invalid_product EXCEPTION;

BEGINUPDATE ProductSET desc= ‘&Product_desc’WHERE ProductId = &Product_No;IF SQL%NOTFOUND THEN

RAISE e_Invalid_Product;END IF;COMMIT;

EXCEPTIONWHEN e_Invalid_Product THEN

DBMS_OUTPUT.PUT_LINE(‘Invalid Product Numbers’);END;


Calling Environments

SQL*PLUS: Displays error number & message to screen

Precompiler Application:

Access error Number via the SQLCA data structure

PL/SQL block:

Traps exception in exception handling block


Example of Propagating Exceptions:DECLARE

a EXCEPTION;b EXCEPTION;PARAGMA EXCEPTION_INIT(b, -2292);

BEGINFOR counter IN fac_cursor LOOPBEGIN

SELECT …UPDATE …IF SQL%NOTFOUND THEN

RAISE a;END IF;

EXCEPTIONWHEN b THEN ….WHEN a THEN ….

END;END LOOP;

EXCEPTIONWHEN NO_DATA_FOUND THEN ….WHEN TOO_MANY_ROWS THEN ….

END;


Exception Propagation:BEGIN

BEGINIF NUM=-1 THEN

RAISE A;ELSIF NUM=1 THEN

RAISE B;ELSE

RAISE C;EXCEPTION

WHEN A THEN…

END;…EXCEPTION

WHEN B THEN…

END;


Declarations Exception:

DECLAREMax CONSTANT NUMBER(2):=999;--

exception

BEGINNULL;

EXCEPTIONWHEN OTHERS THEN

…END;/


Handlers Exception in Exception Clause:

....

EXCEPTIONWHEN INVALID_NUMBER THEN

INSERT INTO …--RAISE DUP_VAL_ON_INDEXWHEN DUP_VAL_ON_INDEX THEN

….END;


PRACTICE:

• Write a procedure to insert name and salary of faculty members whose salary is plus or minus $x (x is passed as a parameter) of the salary, into faculty_stat table with the following exceptions:

• If there is no faculty salary in that range, write a message into logfile table ‘No faculty in $x range’

• If there is more that one faculty, write the sum of faculty members in that range.

• Any other exception should go to the logfile with the appropriate message.


Functions for Trapping Exceptions

SQLCODEReturns the numeric value for the last error

raised

SQLERRMReturns the message associated with the error

number


Example

DECLAREa NUMBER;b VARCHAR2(256);

BEGIN…..

EXCEPTION.….WHEN OTHERS THEN

ROLLBACK;a:= SQLCODE;b:= SQLERRM;INSERT INTO errors VALUES (a, b);

END;


Example

CREATE OR REPLACE PACKAGE dynamicSQLIs

invalid_Table_Name EXCEPTION;PRAGMA EXCEPTION_INIT(invalid_Table_Name, -903);

invalid_Col_Name EXCEPTION;PRAGMA EXCEPTION_INIT(invalid_Col_Name, -904);

.

.

.END dynamicSQL;

To Trap:WHEN dynamicSQL.invalid_Col_Name THEN

……


Example

CREATE OR REPLACE PACKAGE ErrorNo ISError_One CONSTANT NUMBER:=-20050;Exc_One EXCEPTION;PRAGMA EXCEPTION_INIT(Exc_One, Error_One);

Error_Two CONSTANT NUMBER:=-20051;Exc_TwoEXCEPTION;PRAGMA EXCEPTION_INIT(Exc_Two, Error_Two);

END ErrorNo;

PROCEDURE …is ….BEGIN

IF …THEN RAISE_APPLICATION_ERROR(ErrorNo.Error_One, ‘Description of Error’);END IF;


Calling a Procedure or Block to Check for Errors

1. SQL>SET SERVEROUTPUT ON SIZE 1000000 FORMAT TRUNCATED

2. SQL>EXEC deposit(1111, 250, ‘check’ );


Finding the Location of Errors

BEGIN

SELECT …..

SELECT…..

SELECT….

EXCEPTION

……..

END;


Method 1: Finding the location of Errors

BEGINSELECT …..v_counter:=1;SELECT…..v_counter:=2;SELECT….v_counter:=3;

EXCEPTIONWHEN NO_DATA_FOUND THEN

INSERT INTO errorsVALUES (‘Error in statement ‘ || v_counter);

END;


Method 2: Put Each Exception in a Block

BEGINSELECT …..EXCEPTION…

END;BEGIN

SELECT…..EXCEPTION

END;……


CALL STACK

V_callstack VARCHAR2(2000);

BEGIN V_callstack := DBMS_UTILITY.FORMAT_CALL_STACK;


Capture Rows that Cause Errors:

• Create a table called EXCEPTIONS in your schema

• Script is called ‘utlexcpt.sql’ and is in /rdbms/admin directory

• Exceptions table contains four columns:

Row_ID, Owner, Table_Name, Constraint.


Capture Rows that Cause Errors:

Example:--Enable a primary key

ALTER TABLE faculty ENABLE PRIMARY KEYexceptions into exceptions;--If enable failSELECT Owner, Table_Name, constraint FROM exceptions;--What row cause thisSELECT * FROM faculty EHERE Row_ID IN (SELECT Row_ID FROM

EXCEPTIONS);


Retrying a Transaction

BEGINFOR I IN 1…100 LOOPBEGIN

SAVEPOINT a;…DELETE FROM faculty

WHERE age = 65;…INSERT INTO Re_facultyVALUES (name, …);--RAISE DUP_VAL_ON_INDEXCOMMIT;

END;EXCEPTION

WHEN DUP_VAL_ON_INDEX THENROLLBACK TO a;

END;


Predefined Exceptions

Exception Oracle Error SQLCODE Value

CURSOR_ALREADY_OPEN ORA-06511 -6511

INVALID_CURSOR ORA-01001 -1001

NO_DATA_FOUND ORA-01403 +100

SUBSCRIPT_OUTSIDE_LIMIT ORA-06532 -6532

SYS_INVALID_ROWID ORA-01410 -1410

TIMEOUT_ON_RESOURCE ORA-00051 -51

TOO_MANY_ROWS ORA-01422 -1422

ZERO_DIVIDE ORA-01476 -1476

Chapter Eighteen Exception Handling

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