Exceptions are said to be error conditions during a program execution. Programmers use EXCEPTIONS to catch these error conditions in the program. There are two different kinds of exceptions:

1. System-defined Exceptions
2. User-defined Exceptions

Syntax for exception handling:

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;

Raising Exceptions

Database server raises the exceptions automatically whenever, there is an internal database error, but these exceptions are raised explicitly by the programmer by using the command RAISE.

Syntax:
DECLARE 
   exception_name EXCEPTION; 
BEGIN 
   IF condition THEN 
      RAISE exception_name; 
   END IF; 
EXCEPTION 
   WHEN exception_name THEN 
   statement; 
END;

Syntax:
DECLARE
      my_exception EXCEPTION;
Example:
DECLARE 
   t_id teachers.id%type := &tt_id; 
   t_name teacherS.Name%type; 
   t_addr teachers.address%type;  
   -- user defined exception 
   ex_invalid_id  EXCEPTION; 
BEGIN 
   IF t_id <= 0 THEN 
      RAISE ex_invalid_id; 
   ELSE 
      SELECT  name, address INTO  t_name, t_addr 
      FROM teachers 
      WHERE id = t_id;
      DBMS_OUTPUT.PUT_LINE ('Name: '||  t_name);  
      DBMS_OUTPUT.PUT_LINE ('Address: ' || t_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 teacher!'); 

   WHEN others THEN 

      dbms_output.put_line('Error!');  

END; 
/
Output:
Enter value for tt_id: 0 (let's enter a value 0) 
old  2: t_id teachers.id%type := &tt_id; 
new  2: t_id teachers.id%type := 0; 
ID must be greater than zero! 
PL/SQL procedure successfully completed.

PRE-DEFINED EXCEPTIONS

There are pre-defined exceptions, that are executed when any of the database rules are violated by a program.