07- Handling Exceptions


Exception Handling

Exception Handling

In this section we will discuss about the following, 
1) What is Exception Handling. 
2) Structure of Exception Handling. 
3) Types of Exception Handling.

1) What is Exception Handling?

PL/SQL provides a feature to handle the Exceptions which occur in a PL/SQL Block known as exception Handling. Using Exception Handling we can test the code and avoid it from exiting abruptly.

When an exception occurs a messages which explains its cause is received. 
PL/SQL Exception message consists of three parts. 
1) Type of Exception
2) An Error Code
3) A message 

By Handling the exceptions we can ensure a PL/SQL block does not exit abruptly.

2) Structure of Exception Handling.

General Syntax for coding the exception section

 DECLARE
   Declaration section 
 BEGIN 
   Exception section 
 EXCEPTION 
 WHEN ex_name1 THEN 
    -Error handling statements 
 WHEN ex_name2 THEN 
    -Error handling statements 
 WHEN Others THEN 
   -Error handling statements 
END; 

General PL/SQL statments can be used in the Exception Block.

When an exception is raised, Oracle searches for an appropriate exception handler in the exception section. For example in the above example, if the error raised is 'ex_name1 ', then the error is handled according to the statements under it. Since, it is not possible to determine all the possible runtime errors during testing of the code, the 'WHEN Others' exception is used to manage the exceptions that are not explicitly handled. Only one exception can be raised in a Block and the control does not return to the Execution Section after the error is handled.

If there are nested PL/SQL blocks like this.

 DELCARE
   Declaration section 
 BEGIN
    DECLARE
      Declaration section 
    BEGIN 
      Execution section 
    EXCEPTION 
      Exception section 
    END; 
 EXCEPTION
   Exception section 
 END; 

In the above case, if the exception is raised in the inner block it should be handled in the exception block of the inner PL/SQL block else the control moves to the Exception block of the next upper PL/SQL Block. If none of the blocks handle the exception the program ends abruptly with an error.


3) Types of Exception.

There are 3 types of Exceptions. 
a) Named System Exceptions 
b) Unnamed System Exceptions 
c) User-defined Exceptions

a) Named System Exceptions

System exceptions are automatically raised by Oracle, when a program violates a RDBMS rule. There are some system exceptions which are raised frequently, so they are pre-defined and given a name in Oracle which are known as Named System Exceptions.

For example: NO_DATA_FOUND and ZERO_DIVIDE are called Named System exceptions.

Named system exceptions are: 
1) Not Declared explicitly, 
2) Raised implicitly when a predefined Oracle error occurs, 
3) caught by referencing the standard name within an exception-handling routine.

Exception NameReasonError 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

For Example: Suppose a NO_DATA_FOUND exception is raised in a proc, we can write a code to handle the exception as given below.

BEGIN 
  Execution section
EXCEPTION 
WHEN NO_DATA_FOUND THEN 
 dbms_output.put_line ('A SELECT...INTO did not return any row.'); 
 END; 

b) Unnamed System Exceptions

Those system exception for which oracle does not provide a name is known as unnamed system exception. These exception 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.

We can assign a name to unnamed system exceptions using a Pragma called EXCEPTION_INIT. 
EXCEPTION_INIT will associate a predefined Oracle error number to a programmer_defined exception name.

Steps to be followed to use unnamed system exceptions are 
• They are raised implicitly. 
• If they are not handled in WHEN Others they must be handled explicity. 
• To handle the exception explicity, they must be declared using Pragma EXCEPTION_INIT as given above and handled referencing the user-defined exception name in the exception section.

The general syntax to declare unnamed system exception using EXCEPTION_INIT is:

DECLARE 
   exception_name EXCEPTION; 
   PRAGMA 
   EXCEPTION_INIT (exception_name, Err_code); 
BEGIN 
Execution section
EXCEPTION
  WHEN exception_name THEN
     handle the exception
END;

For Example: Lets consider the product table and order_items table from sql joins.

Here product_id is a primary key in product table and a foreign key in order_items table. 
If we try to delete a product_id from the product table when it has child records in order_id table an exception will be thrown with oracle code number -2292. 
We can provide a name to this exception and handle it in the exception section as given below.

 DECLARE 
  Child_rec_exception EXCEPTION; 
  PRAGMA 
   EXCEPTION_INIT (Child_rec_exception, -2292); 
BEGIN 
  Delete FROM product where product_id= 104; 
EXCEPTION 
   WHEN Child_rec_exception 
   THEN Dbms_output.put_line('Child records are present for this product_id.'); 
END; 
/ 

c) User-defined Exceptions

Apart from system exceptions we can explicity define exceptions based on business rules. These are known as user-defined exceptions.

Steps to be followed to use 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.

For Example: Lets consider the product table and order_items table from sql joins to explain user-defined exception. 
Lets create a business rule that if the total no of units of any particular product sold is more than 20, then it is a huge quantity and a special discount should be provided.

DECLARE 
  huge_quantity EXCEPTION; 
  CURSOR product_quantity is 
  SELECT p.product_name as name, sum(o.total_units) as units
  FROM order_tems o, product p
  WHERE o.product_id = p.product_id; 
  quantity order_tems.total_units%type; 
  up_limit CONSTANT order_tems.total_units%type := 20; 
  message VARCHAR2(50); 
BEGIN 
  FOR product_rec in product_quantity LOOP 
    quantity := product_rec.units;
     IF quantity > up_limit THEN 
      message := 'The number of units of product ' || product_rec.name ||  
                 ' is more than 20. Special discounts should be provided. 
		 Rest of the records are skipped. '
     RAISE huge_quantity; 
     ELSIF quantity < up_limit THEN 
      v_message:= 'The number of unit is below the discount limit.'; 
     END IF; 
     dbms_output.put_line (message); 
  END LOOP; 
 EXCEPTION 
   WHEN huge_quantity THEN 
     dbms_output.put_line (message); 
 END; 
/