Update by after insert trigger

Question

I need to update each new inserted row of data by checking if there were same values in the table. So I tried the After Insert Trigger with the code below :
CREATE OR REPLACE TRIGGER approve_checker
after insert on TBL_BILL
for each row
declare
counter integer;
begin
SELECT COUNT() INTO counter FROM TBL_BILL WHERE issue_date=:NEW.issue_date AND wagon_no=:NEW.wagon_no ;*
IF( counter > 1)
THEN
UPDATE TBL_BILL SET approved = 2 WHERE id = :NEW.id;
END IF;
end approve_checker;
But errors accured :
ORA-04091: table RAIL.BILL is mutating, trigger/function may not see it
ORA-06512: at "RAIL.APPROVE_CHECKER", line 4
ORA-04088: error during execution of trigger 'RAIL.APPROVE_CHECKER'
Any help appreciated.

Answer 1

You cannot SELECT from the table on which the row-level trigger is defined, nor can you UPDATE a table on which the row-level trigger is defined. If you are inserting multiple rows in a single INSERT statement, Oracle can't be sure what rows to update and/or select.
Is there a reason that you wouldn't just create a unique constraint to enforce this condition?
Justin