Thursday, 14 June 2012

PLSQL LEARNING CLASS8



ERROR HANDLING

PL/SQL implements error handling with exceptions and exception handlers. Exceptions can be associated with oracle errors or with your own user-defined errors. By using exceptions and exception handlers, you can make your PL/SQL programs robust and able to deal with both unexpected and expected errors during execution.

ERROR TYPES

  • Compile-time errors
  • Runtime errors

Errors that occur during the compilation phase are detected by the PL/SQL engine and reported back to the user, we have to correct them.
Runtime errors are detected by the PL/SQL runtime engine which can programmatically raise and caught by exception handlers.
Exceptions are designed for run-time error handling, rather than compile-time error handling.

HANDLING EXCEPTIONS

When exception is raised, control passes to the exception section of the block. The exception section consists of handlers for some or all of the exceptions. An exception handler contains the code that is executed when the error associated with the exception occurs, and the exception is raised.

Syntax:
EXCEPTION
When exception_name then
Sequence_of_statements;
When exception_name then
Sequence_of_statements;
When others then
Sequence_of_statements;
END;
EXCEPTION TYPES

  • Predefined exceptions
  • User-defined exceptions

PREDEFINED EXCEPTIONS

Oracle has predefined several exceptions that corresponds to the most common oracle errors. Like the predefined types, the identifiers of these exceptions are defined in the STANDARD package. Because of this, they are already available to the program, it is not necessary to declare them in the declarative secion.

Ex1:
DECLARE
a number;
b varchar(2);
v_marks number;
cursor c is select * from student;
type t is varray(3) of varchar(2);
va t := t('a','b');
va1 t;
BEGIN
-- NO_DATA_FOUND
BEGIN
select smarks into v_marks from student where sno = 50;
EXCEPTION
when no_data_found then
dbms_output.put_line('Invalid student number');
END;
-- CURSOR_ALREADY_OPEN
BEGIN
open c;
open c;
EXCEPTION
when cursor_already_open then
dbms_output.put_line('Cursor is already opened');
END;

-- INVALID_CURSOR
BEGIN
close c;
open c;
close c;
close c;
EXCEPTION
when invalid_cursor then
dbms_output.put_line('Cursor is already closed');
END;
-- TOO_MANY_ROWS
BEGIN
select smarks into v_marks from student where sno > 1;
EXCEPTION
when too_many_rows then
dbms_output.put_line('Too many values are coming to marks variable');
END;
-- ZERO_DIVIDE
BEGIN
a := 5/0;
EXCEPTION
when zero_divide then
dbms_output.put_line('Divided by zero - invalid operation');
END;
-- VALUE_ERROR
BEGIN
b := 'saketh';
EXCEPTION
when value_error then
dbms_output.put_line('Invalid string length');
END;
-- INVALID_NUMBER
BEGIN
insert into student values('a','srinu',100);
EXCEPTION
when invalid_number then
dbms_output.put_line('Invalid number');
END;
-- SUBSCRIPT_OUTSIDE_LIMIT
BEGIN
va(4) := 'c';
EXCEPTION
when subscript_outside_limit then
dbms_output.put_line('Index is greater than the limit');
END;
-- SUBSCRIPT_BEYOND_COUNT
BEGIN
va(3) := 'c';
EXCEPTION
when subscript_beyond_count then
dbms_output.put_line('Index is greater than the count');
END;
-- COLLECTION_IS_NULL
BEGIN
va1(1) := 'a';
EXCEPTION
when collection_is_null then
dbms_output.put_line('Collection is empty');
END;
--
END;

Output:
Invalid student number
Cursor is already opened
Cursor is already closed
Too many values are coming to marks variable
Divided by zero - invalid operation
Invalid string length
Invalid number
Index is greater than the limit
Index is greater than the count
Collection is empty

Ex2:
DECLARE
c number;
BEGIN
c := 5/0;
EXCEPTION
when zero_divide then
dbms_output.put_line('Invalid Operation');
when others then
dbms_output.put_line('From OTHERS handler: Invalid Operation');
END;

Output:
Invalid Operation

USER-DEFINED EXCEPTIONS

A user-defined exception is an error that is defined by the programmer. User-defined exceptions are declared in the declarative secion of a PL/SQL block. Just like variables, exeptions have a type EXCEPTION and scope.

RAISING EXCEPTIONS

User-defined exceptions are raised explicitly via the RAISE statement.
Ex:
DECLARE
e exception;
BEGIN
raise e;
EXCEPTION
when e then
dbms_output.put_line('e is raised');
END;
Output:
e is raised

BULIT-IN ERROR FUNCTIONS

SQLCODE AND SQLERRM

SQLCODE returns the current error code, and SQLERRM returns the current error message text;
For user-defined exception SQLCODE returns 1 and SQLERRM returns “user-deifned exception”.
SQLERRM wiil take only negative value except 100. If any positive value other than 100 returns non-oracle exception.
Ex1:
DECLARE
e exception;
v_dname varchar(10);
BEGIN
-- USER-DEFINED EXCEPTION
BEGIN
raise e;
EXCEPTION
when e then
dbms_output.put_line(SQLCODE || ' ' || SQLERRM);
END;

-- PREDEFINED EXCEPTION
BEGIN
select dname into v_dname from dept where deptno = 50;
EXCEPTION
when no_data_found then
dbms_output.put_line(SQLCODE || ' ' || SQLERRM);
END;
END;

Output:
1 User-Defined Exception
100 ORA-01403: no data found

Ex2:
BEGIN
dbms_output.put_line(SQLERRM(100));
dbms_output.put_line(SQLERRM(0));
dbms_output.put_line(SQLERRM(1));
dbms_output.put_line(SQLERRM(-100));
dbms_output.put_line(SQLERRM(-500));
dbms_output.put_line(SQLERRM(200));
dbms_output.put_line(SQLERRM(-900));
END;
Output:
ORA-01403: no data found
ORA-0000: normal, successful completion
User-Defined Exception
ORA-00100: no data found
ORA-00500: Message 500 not found; product=RDBMS; facility=ORA
-200: non-ORACLE exception
ORA-00900: invalid SQL statement

DBMS_UTILITY.FORMAT_ERROR_STACK

The built-in function, like SQLERRM, returns the message associated with the current error.
It differs from SQLERRM in two ways:
Its length is not restricted; it will return the full error message string.
You can not pass an error code number to this function; it cannot be used to return the message for a random error code.
Ex:
DECLARE
v number := 'ab';
BEGIN
null;
EXCEPTION
when others then
dbms_output.put_line(dbms_utility.format_error_stack);
END;
Output:
declare
*
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: character to number conversion error
ORA-06512: at line 2

DBMS_UTILITY.FORMAT_CALL_STACK

This function returns a formatted string showing the execution call stack inside your PL/SQL application. Its usefulness is not restricted to error management; you will also find its handy for tracing the exectution of your code. You may not use this function in exception block.
Ex:
BEGIN
dbms_output.put_line(dbms_utility.format_call_stack);
END;
Output:
----- PL/SQL Call Stack -----
Object_handle line_number object_name
69760478 2 anonymous block

DBMS_UTILITY.FORMAT_ERROR_BACKTRACE

It displays the execution stack at the point where an exception was raised. Thus , you can call this function with an exception section at the top level of your stack and still find out where the error was raised deep within the call stack.
Ex:
CREATE OR REPLACE PROCEDURE P1 IS
BEGIN
dbms_output.put_line('from procedure 1');
raise value_error;
END P1;

CREATE OR REPLACE PROCEDURE P2 IS
BEGIN
dbms_output.put_line('from procedure 2');
p1;
END P2;

CREATE OR REPLACE PROCEDURE P3 IS
BEGIN
dbms_output.put_line('from procedure 3');
p2;
EXCEPTION
when others then
dbms_output.put_line(dbms_utility.format_error_backtrace);
END P3;
Output:
SQL> exec p3

from procedure 3
from procedure 2
from procedure 1
ORA-06512: at "SAKETH.P1", line 4
ORA-06512: at "SAKETH.P2", line 4
ORA-06512: at "SAKETH.P3", line 4

EXCEPTION_INIT PRAGMA

Using this you can associate a named exception with a particular oracle error. This gives you the ability to trap this error specifically, rather than via an OTHERS handler.
Syntax:
PRAGMA EXCEPTION_INIT(exception_name, oracle_error_number);
Ex:
DECLARE
e exception;
pragma exception_init(e,-1476);
c number;
BEGIN
c := 5/0;
EXCEPTION
when e then
dbms_output.put_line('Invalid Operation');
END;

Output:
Invalid Operation

RAISE_APPLICATION_ERROR

You can use this built-in function to create your own error messages, which can be more descriptive than named exceptions.
Syntax:
RAISE_APPLICATION_ERROR(error_number, error_message,, [keep_errors_flag]);
The Boolean parameter keep_errors_flag is optional. If it is TRUE, the new error is added to the list of errors already raised. If it is FALSE, which is default, the new error will replace the current list of errors.
Ex:
DECLARE
c number;
BEGIN
c := 5/0;

EXCEPTION
when zero_divide then
raise_application_error(-20222,'Invalid Operation');
END;
Output:
DECLARE
*
ERROR at line 1:
ORA-20222: Invalid Operation
ORA-06512: at line 7

EXCEPTION PROPAGATION

Exceptions can occur in the declarative, the executable, or the exception section of a PL/SQL block.

EXCEPTION RAISED IN THE EXECUATABLE SECTION

Exceptions raised in execuatable section can be handled in current block or outer block.
Ex1:
DECLARE
e exception;
BEGIN
BEGIN
raise e;
END;
EXCEPTION
when e then
dbms_output.put_line('e is raised');
END;
Output:
e is raised

Ex2:
DECLARE
e exception;
BEGIN
BEGIN
raise e;
END;
END;
Output:
ERROR at line 1:
ORA-06510: PL/SQL: unhandled user-defined exception
ORA-06512: at line 5

EXCEPTION RAISED IN THE DECLARATIVE SECTION

Exceptions raised in the declarative secion must be handled in the outer block.
Ex1:
DECLARE
c number(3) := 'abcd';
BEGIN
dbms_output.put_line('Hello');
EXCEPTION
when others then
dbms_output.put_line('Invalid string length');
END;

Output:
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: character to number conversion error
ORA-06512: at line 2

Ex2:
BEGIN
DECLARE
c number(3) := 'abcd';
BEGIN
dbms_output.put_line('Hello');
EXCEPTION
when others then
dbms_output.put_line('Invalid string length');
END;
EXCEPTION
when others then
dbms_output.put_line('From outer block: Invalid string length');
END;

Output:
From outer block: Invalid string length

EXCEPTION RAISED IN THE EXCEPTION SECTION

Exceptions raised in the declarative secion must be handled in the outer block.
Ex1:
DECLARE
e1 exception;
e2 exception;
BEGIN
raise e1;
EXCEPTION
when e1 then
dbms_output.put_line('e1 is raised');
raise e2;
when e2 then
dbms_output.put_line('e2 is raised');
END;

Output:
e1 is raised
DECLARE
*
ERROR at line 1:
ORA-06510: PL/SQL: unhandled user-defined exception
ORA-06512: at line 9
ORA-06510: PL/SQL: unhandled user-defined exception

Ex2:
DECLARE
e1 exception;
e2 exception;
BEGIN
BEGIN
raise e1;
EXCEPTION
when e1 then
dbms_output.put_line('e1 is raised');
raise e2;
when e2 then
dbms_output.put_line('e2 is raised');
END;
EXCEPTION
when e2 then
dbms_output.put_line('From outer block: e2 is raised');
END;

Output:
e1 is raised
From outer block: e2 is raised

Ex3:
DECLARE
e exception;
BEGIN
raise e;
EXCEPTION
when e then
dbms_output.put_line('e is raised');
raise e;
END;

Output:
e is raised
DECLARE
*
ERROR at line 1:
ORA-06510: PL/SQL: unhandled user-defined exception
ORA-06512: at line 8
ORA-06510: PL/SQL: unhandled user-defined exception

RESTRICTIONS

You can not pass exception as an argument to a subprogram.


















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