Exception Handling in PL/SQL

1. User defined exception.
2. System defined exceptions.

WHEN exception THEN 
    statement;

DECLARE declarations section; BEGIN executable command(s); EXCEPTION WHEN exception1 THEN statement1; WHEN exception2 THEN statement2; [WHEN others THEN] /* default exception handling code */ END;

1. System defined exceptions: These exceptions are predefined in PL/SQL which get raised WHEN certain database rule is violated. System-defined exceptions are further divided into two categories:
   1. Named system exceptions.
   2. Unnamed system exceptions.
   • Named system exceptions: They have a predefined name by the system like ACCESS_INTO_NULL, DUP_VAL_ON_INDEX, LOGIN_DENIED etc. the list is quite big. So we will discuss some of the most commonly used exceptions: Lets create a table geeks.

     create table geeks(g_id int , g_name varchar(20), marks int); 
     insert into geeks values(1, 'Suraj',100);
     insert into geeks values(2, 'Praveen',97);
     insert into geeks values(3, 'Jessie', 99);
     

     
     1. NO_DATA_FOUND: It is raised WHEN a SELECT INTO statement returns no rows. For eg: SQL

        DECLARE
           temp varchar(20);
        
        BEGIN
           SELECT g_id into temp from geeks where g_name='GeeksforGeeks';
        
        exception
           WHEN no_data_found THEN
              dbms_output.put_line('ERROR');
              dbms_output.put_line('there is no name as');
              dbms_output.put_line('GeeksforGeeks in geeks table');
        end;
        

        Output:

        ERROR
        there is no name as GeeksforGeeks in geeks table
        

     2. TOO_MANY_ROWS:It is raised WHEN a SELECT INTO statement returns more than one row. SQL

        DECLARE
           temp varchar(20);
        
        BEGIN
        
        -- raises an exception as SELECT 
        -- into trying to return too many rows
           SELECT g_name into temp from geeks;
           dbms_output.put_line(temp);
        
        EXCEPTION
           WHEN too_many_rows THEN
              dbms_output.put_line('error trying to SELECT too many rows');
        
        end;
        

        Output:

        error trying to SELECT too many rows
        

     3. VALUE_ERROR:This error is raised WHEN a statement is executed that resulted in an arithmetic, numeric, string, conversion, or constraint error. This error mainly results from programmer error or invalid data input. SQL

        DECLARE
           temp number;   
        
        BEGIN
           SELECT g_name  into temp from geeks where g_name='Suraj';
           dbms_output.put_line('the g_name is '||temp);
        
        EXCEPTION
           WHEN value_error THEN
           dbms_output.put_line('Error');
           dbms_output.put_line('Change data type of temp to varchar(20)');
        
        END;
        

        Output:

        Error
        Change data type of temp to varchar(20)
        

     4. ZERO_DIVIDE = raises exception WHEN dividing with zero. SQL

        DECLARE
           a int:=10;
           b int:=0;
           answer int;
        
        BEGIN
           answer:=a/b;
           dbms_output.put_line('the result after division is'||answer);
        
        exception
           WHEN zero_divide THEN
              dbms_output.put_line('dividing by zero please check the values again');
              dbms_output.put_line('the value of a is '||a);
              dbms_output.put_line('the value of b is '||b);
        END;
        

        Output:

        dividing by zero please check the values again
        the value of a is 10
        the value of b is 0
        

   
   
   • Unnamed system exceptions:Oracle doesn't provide name for some system exceptions called unnamed system exceptions.These exceptions don't occur frequently.These exceptions have two parts code and an associated message. The way to handle to these exceptions is to assign name to them using Pragma EXCEPTION_INIT Syntax:

     PRAGMA EXCEPTION_INIT(exception_name, -error_number);
     

     error_number are pre-defined and have negative integer range from -20000 to -20999. Example: SQL

     DECLARE
        exp exception;
        pragma exception_init (exp, -20015);
        n int:=10;
     
     BEGIN 
        FOR i IN 1..n LOOP
           dbms_output.put_line(i*i);
              IF i*i=36 THEN
                 RAISE exp;
              END IF;
        END LOOP;
     
     EXCEPTION
        WHEN exp THEN
           dbms_output.put_line('Welcome to GeeksforGeeks');
     
     END;
     

     Output:

     1
     4
     9
     16
     25
     36
     Welcome to GeeksforGeeks
     

   
   
2. User defined exceptions: This type of users can create their own exceptions according to the need and to raise these exceptions explicitly raise command is used. Example:
   • Divide non-negative integer x by y such that the result is greater than or equal to 1. From the given question we can conclude that there exist two exceptions
     • Division be zero.
     • If result is greater than or equal to 1 means y is less than or equal to x.
     SQL

     DECLARE
        x int:=&x; /*taking value at run time*/
        y int:=&y;
        div_r float;
        exp1 EXCEPTION;
        exp2 EXCEPTION;
     
     BEGIN
        IF y=0 then
            raise exp1;
     
        ELSEIF y > x then
           raise exp2;
     
        ELSE
           div_r:= x / y;
           dbms_output.put_line('the result is '||div_r);
     
        END IF;
     
     EXCEPTION
        WHEN exp1 THEN
           dbms_output.put_line('Error');
           dbms_output.put_line('division by zero not allowed');
     
        WHEN exp2 THEN
           dbms_output.put_line('Error');
           dbms_output.put_line('y is greater than x please check the input');
     
     END;
     

     Input 1: x = 20
              y = 10
     
     Output: the result is 2
     

     Input 2: x = 20
              y = 0
     
     Output:
     Error
     division by zero not allowed
     

     Input 3: x=20
              y = 30
     
     Output:<.em>
     Error
     y is greater than x please check the input
     

   
   RAISE_APPLICATION_ERROR: It is used to display user-defined error messages with error number whose range is in between -20000 and -20999. When RAISE_APPLICATION_ERROR executes it returns error message and error code which looks same as Oracle built-in error. Example: SQL

   DECLARE
       myex EXCEPTION;
       n NUMBER :=10;
   
   BEGIN
       FOR i IN 1..n LOOP
       dbms_output.put_line(i*i);
           IF i*i=36 THEN
           RAISE myex;
           END IF;
       END LOOP;
   
   EXCEPTION
       WHEN myex THEN
           RAISE_APPLICATION_ERROR(-20015, 'Welcome to GeeksForGeeks');
   
   END;
   

   Output:

   Error report:
   ORA-20015: Welcome to GeeksForGeeks
   ORA-06512: at line 13
   
   1
   4
   9
   16
   25
   36
   
   
   

   Note: The output is based on Oracle Sql developer, the output order might change IF you're running this code somewhere else. Scope rules in exception handling:
   1. We can't DECLARE an exception twice but we can DECLARE the same exception in two dIFferent blocks.
   2. Exceptions DECLAREd inside a block are local to that block and global to all its sub-blocks.
   As a block can reference only local or global exceptions, enclosing blocks cannot reference exceptions DECLAREd in a sub-block. If we reDECLARE a global exception in a sub-block, the local declaration prevails i.e. the scope of local is more. Example: SQL

   DECLARE
      GeeksforGeeks EXCEPTION;
      age NUMBER:=16;
   BEGIN
   
      --  sub-block BEGINs 
      DECLARE       
         
         -- this declaration prevails 
         GeeksforGeeks  EXCEPTION;  
         age NUMBER:=22;
     
      BEGIN
         IF age > 16 THEN
            RAISE GeeksforGeeks; /* this is not handled*/
         END IF;
      
      END;          
      -- sub-block ends
   
   EXCEPTION
     -- Does not handle raised exception 
     WHEN GeeksforGeeks THEN
       DBMS_OUTPUT.PUT_LINE
         ('Handling  GeeksforGeeks exception.');
     
     WHEN OTHERS THEN
       DBMS_OUTPUT.PUT_LINE
         ('Could not recognize exception GeeksforGeeks  in this scope.');
   END;
   

   Output:

   Could not recognize exception GeeksforGeeks  in this scope.