Live ORACLE

DECLARE
 num_tables  NUMBER;
BEGIN
 SELECT Count(* )
 INTO   num_tables
 FROM   user_tables;
 IF num_tables < 1000 THEN
   /* Issue your own error code (ORA-20101) with your own error message.
   Note that you do not need to qualify raise_application_error with
   DBMS_STANDARD */
   Raise_application_error(-20101,'Expecting at least 1000 tables');
 ELSE
   NULL; -- Do the rest of the processing (for the non-error case).
 END IF;
END;
/  

DECLARE
*
ERROR at line 1:
ORA-20101: Expecting at least 1000 tables
ORA-06512: at line 9

Raising an Application Error With raise_application_error