SQL> create table test
  2  (  appl_id      number,
  3     start_date   date
  4  );

Table created.

SQL> create or replace trigger trg_biu_test
  2     before insert or update
  3     on test
  4     for each row
  5  declare
  6     l_max_date   date;
  7  begin
  8     select max (t.start_date)
  9       into l_max_date
 10       from test t
 11      where t.appl_id = :new.appl_id;
 12
 13     if months_between(:new.start_date, l_max_date) < 6 then
 14        raise_application_error (-20001, '6 months between two dates are required');
 15     end if;
 16  end;
 17  /

Trigger created.

SQL>

SQL> insert into test (appl_id, start_date) values (1, date '2013-01-15');

1 row created.

SQL> insert into test (appl_id, start_date) values (1, date '2013-04-15');
insert into test (appl_id, start_date) values (1, date '2013-04-15')
            *
ERROR at line 1:
ORA-20001: 6 months between two dates are required
ORA-06512: at "SCOTT.TRG_BIU_TEST", line 10
ORA-04088: error during execution of trigger 'SCOTT.TRG_BIU_TEST'


SQL>

SQL> insert into test (appl_id, start_date) values (1, date '2013-08-15');

1 row created.

SQL> update test set appl_id = 2;
 
update test set appl_id = 2
 
ORA-04091: table LIVE.TEST is mutating, trigger/function may not see it
ORA-06512: at "LIVE.TRG_BIU_TEST", line 4
ORA-04088: error during execution of trigger 'LIVE.TRG_BIU_TEST'
 
SQL> insert into test select rownum, sysdate from dual connect by level < 3;
 
insert into test select rownum, sysdate from dual connect by level < 3
 
ORA-04091: table LIVE.TEST is mutating, trigger/function may not see it
ORA-06512: at "LIVE.TRG_BIU_TEST", line 4
ORA-04088: error during execution of trigger 'LIVE.TRG_BIU_TEST'
 
SQL> 

 create table test
      (dt_start DATE,
       dt_end DATE,
       constraint chk_strt check (dt_end >= dt_start));


Insert into test VALUES (sysdate, sysdate + 6);

where :apl_nmbr = :apl_nmbr

where apl_nmbr = :apl_nmbr

CREATE OR REPLACE TRIGGER trg_biu_test
   BEFORE INSERT OR UPDATE OF start_date
   ON TESTT
   FOR EACH ROW
DECLARE
   l_max_date   DATE;
BEGIN
   SELECT MAX (t.start_date)
     INTO l_max_date
     FROM TESTT t
    WHERE t.appl_id = :NEW.appl_id;

   IF MONTHS_BETWEEN (:NEW.start_date, l_max_date) < 6
   THEN
      raise_application_error (-20001,
                               '6 months between two dates are required'
                              );
   END IF;
END;