An error condition during a program execution is called an exception in PL/SQL. PL/SQL supports programmers to catch such conditions using EXCEPTION block in the program and an appropriate action is taken against the error condition. There are two types of exceptions:
- System-defined exceptions
- User-defined exceptions
Syntax for Exception Handling
The General Syntax for exception handling is as follows. Here you can list down as many as exceptions you want to handle. The default exception will be handled using WHEN others THEN:
DECLARE <declarations section> BEGIN <executable command(s)> EXCEPTION <exception handling goes here > WHEN exception1 THEN exception1-handling-statements WHEN exception2 THEN exception2-handling-statements WHEN exception3 THEN exception3-handling-statements ........ WHEN others THEN exception3-handling-statements END;
Example
Let us write some simple code to illustrate the concept. We will be using the CUSTOMERS table we had created and used in the previous chapters:
DECLARE c_id customers.id%type := 8; c_name customers.name%type; c_addr customers.address%type; BEGIN SELECT name, address INTO c_name, c_addr FROM customers WHERE id = c_id; DBMS_OUTPUT.PUT_LINE ('Name: '|| c_name); DBMS_OUTPUT.PUT_LINE ('Address: ' || c_addr); EXCEPTION WHEN no_data_found THEN dbms_output.put_line('No such customer!'); WHEN others THEN dbms_output.put_line('Error!'); END; /
When the above code is executed at SQL prompt, it produces the following result:
No such customer! PL/SQL procedure successfully completed.
The above program displays the name and address of a customer whose ID is given. Since there is no customer with ID value 8 in our database, the program raises the run-time exception NO_DATA_FOUND, which is captured in EXCEPTION block.
Raising Exceptions
Exceptions are raised by the database server automatically whenever there is any internal database error, but exceptions can be raised explicitly by the programmer by using the command RAISE. Following is the simple syntax of raising an exception:
DECLARE exception_name EXCEPTION; BEGIN IF condition THEN RAISE exception_name; END IF; EXCEPTION WHEN exception_name THEN statement; END;
You can use above syntax in raising Oracle standard exception or any user-defined exception. Next section will give you an example on raising user-defined exception, similar way you can raise Oracle standard exceptions as well.
User-defined Exceptions
PL/SQL allows you to define your own exceptions according to the need of your program. A user-defined exception must be declared and then raised explicitly, using either a RAISE statement or the procedure DBMS_STANDARD.RAISE_APPLICATION_ERROR.
The syntax for declaring an exception is:
DECLARE my-exception EXCEPTION;
Example:
The following example illustrates the concept. This program asks for a customer ID, when the user enters an invalid ID, the exception invalid_id is raised.
DECLARE c_id customers.id%type := &cc_id; c_name customers.name%type; c_addr customers.address%type; -- user defined exception ex_invalid_id EXCEPTION; BEGIN IF c_id <= 0 THEN RAISE ex_invalid_id; ELSE SELECT name, address INTO c_name, c_addr FROM customers WHERE id = c_id; DBMS_OUTPUT.PUT_LINE ('Name: '|| c_name); DBMS_OUTPUT.PUT_LINE ('Address: ' || c_addr); END IF; EXCEPTION WHEN ex_invalid_id THEN dbms_output.put_line('ID must be greater than zero!'); WHEN no_data_found THEN dbms_output.put_line('No such customer!'); WHEN others THEN dbms_output.put_line('Error!'); END; /
When the above code is executed at SQL prompt, it produces the following result:
Enter value for cc_id: -6 (let's enter a value -6) old 2: c_id customers.id%type := &cc_id; new 2: c_id customers.id%type := -6; ID must be greater than zero! PL/SQL procedure successfully completed.
Pre-defined Exceptions
PL/SQL provides many pre-defined exceptions, which are executed when any database rule is violated by a program. For example, the predefined exception NO_DATA_FOUND is raised when a SELECT INTO statement returns no rows. The following table lists few of the important pre-defined exceptions:
| Exception | Oracle Error | SQLCODE | 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 s 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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