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PL/SQL Exception Handling

What is Exception

An error occurs during the program execution is called Exception in PL/SQL.

PL/SQL facilitates programmers to catch such conditions using exception block in the program and an appropriate action is taken against the error condition.

There are two type of exceptions:

  • System-defined Exceptions
  • User-defined Exceptions

PL/SQL Exception Handling

Syntax for exception handling:

Following is a general syntax for exception handling:

  1. DECLARE
  2.    <declarations section>
  3. BEGIN
  4.    <executable command(s)>
  5. EXCEPTION
  6.    <exception handling goes here >
  7.    WHEN exception1 THEN
  8.        exception1-handling-statements
  9.    WHEN exception2  THEN
  10.       exception2-handling-statements
  11.    WHEN exception3 THEN
  12.       exception3-handling-statements
  13.    ……..
  14.    WHEN others THEN
  15.       exception3-handling-statements
  16. END;

Example of exception handling

Let’s take a simple example to demonstrate the concept of exception handling. Here we are using the already created CUSTOMERS table.

SELECT* FROM COUSTOMERS;

ID NAME AGE ADDRESS SALARY
1 Ramesh 23 Allahabad 20000
2 Suresh 22 Kanpur 22000
3 Mahesh 24 Ghaziabad 24000
4 Chandan 25 Noida 26000
5 Alex 21 Paris 28000
6 Sunita 20 Delhi 30000
  1. DECLARE
  2.    c_id customers.id%type := 8;
  3.    c_name  customers.name%type;
  4.    c_addr customers.address%type;
  5. BEGIN
  6.    SELECT  name, address INTO  c_name, c_addr
  7.    FROM customers
  8.    WHERE id = c_id;
  9. DBMS_OUTPUT.PUT_LINE (‘Name: ‘||  c_name);
  10.  DBMS_OUTPUT.PUT_LINE (‘Address: ‘ || c_addr);
  11. EXCEPTION
  12.    WHEN no_data_found THEN
  13.       dbms_output.put_line(‘No such customer!’);
  14.    WHEN others THEN
  15.       dbms_output.put_line(‘Error!’);
  16. END;
  17. /

After the execution of above code at SQL Prompt, it produces the following result:

No such customer!
PL/SQL procedure successfully completed.

The above program should show the name and address of a customer as result whose ID is given. But there is no customer with ID value 8 in our database, so the program raises the run-time exception NO_DATA_FOUND, which is captured in EXCEPTION block.

Note: You get the result “No such customer” because the customer_id used in the above example is 8 and there is no cutomer having id value 8 in that table.

If you use the id defined in the above table (i.e. 1 to 6), you will get a certain result. For a demo example: here, we are using the id 5.

  1. DECLARE
  2.    c_id customers.id%type := 5;
  3.    c_name  customers.name%type;
  4.    c_addr customers.address%type;
  5. BEGIN
  6.    SELECT  name, address INTO  c_name, c_addr
  7.    FROM customers
  8.    WHERE id = c_id;
  9. DBMS_OUTPUT.PUT_LINE (‘Name: ‘||  c_name);
  10.  DBMS_OUTPUT.PUT_LINE (‘Address: ‘ || c_addr);
  11. EXCEPTION
  12.    WHEN no_data_found THEN
  13.       dbms_output.put_line(‘No such customer!’);
  14.    WHEN others THEN
  15.       dbms_output.put_line(‘Error!’);
  16. END;
  17. /

After the execution of above code at SQL prompt, you will get the following result:

Name: alex
Address: paris
PL/SQL procedure successfully completed.

Raising Exceptions

In the case of any internal database error, exceptions are raised by the database server automatically. But it can also be raised explicitly by programmer by using command RAISE.

Syntax for raising an exception:

  1. DECLARE
  2.    exception_name EXCEPTION;
  3. BEGIN
  4.    IF condition THEN
  5.       RAISE exception_name;
  6.    END IF;
  7. EXCEPTION
  8.    WHEN exception_name THEN
  9.    statement;
  10. END;

PL/SQL User-defined Exceptions

PL/SQL facilitates their users to define their own exceptions according to the need of the program. A user-defined exception can be raised explicitly, using either a RAISE statement or the procedure DBMS_STANDARD.RAISE_APPLICATION_ERROR.

Syntax for user define exceptions

  1. DECLARE
  2. my-exception EXCEPTION;

PL/SQL Pre-defined Exceptions

There are many pre-defined exception in PL/SQL which are executed when any database rule is violated by the programs.

For example: NO_DATA_FOUND is a pre-defined exception which is raised when a SELECT INTO statement returns no rows.

Following is a list of some important pre-defined exceptions:

Exception Oracle Error SQL Code Description
ACCESS_INTO_NULL 06530 -6530 It is raised when a NULL object is automatically assigned a value.
CASE_NOT_FOUND 06592 -6592 It is raised when none of the choices in the ?WHEN? clauses of a CASE statement is selected, and there is no else clause.
COLLECTION_IS_NULL 06531 -6531 It is raised when a program attempts to apply collection methods other than exists to an uninitialized nested table or varray, or the program attempts to assign values to the elements of an uninitialized nested table or varray.
DUP_VAL_ON_INDEX 00001 -1 It is raised when duplicate values are attempted to be stored in a column with unique index.
INVALID_CURSOR 01001 -1001 It is raised when attempts are made to make a cursor operation that is not allowed, such as closing an unopened cursor.
INVALID_NUMBER 01722 -1722 It is raised when the conversion of a character string into a number fails because the string does not represent a valid number.
LOGIN_DENIED 01017 -1017 It is raised when s program attempts to log on to the database with an invalid username or password.
NO_DATA_FOUND 01403 +100 It is raised when a select into statement returns no rows.
NOT_LOGGED_ON 01012 -1012 It is raised when a database call is issued without being connected to the database.
PROGRAM_ERROR 06501 -6501 It is raised when PL/SQL has an internal problem.
ROWTYPE_MISMATCH 06504 -6504 It is raised when a cursor fetches value in a variable having incompatible data type.
SELF_IS_NULL 30625 -30625 It is raised when a member method is invoked, but the instance of the object type was not initialized.
STORAGE_ERROR 06500 -6500 It is raised when PL/SQL ran out of memory or memory was corrupted.
TOO_MANY_ROWS 01422 -1422 It is raised when a SELECT INTO statement returns more than one row.
VALUE_ERROR 06502 -6502 It is raised when an arithmetic, conversion, truncation, or size-constraint error occurs.
ZERO_DIVIDE 01476 1476 It is raised when an attempt is made to divide a number by zero.
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