How can I solve the mutating table error here? - oracle

Question

set serveroutput on;
CREATE OR REPLACE TRIGGER hw3
BEFORE DELETE on ENROLLS
for EACH ROW
ENABLE
DECLARE
v_sid number;
v_term varchar2(20);
v_sectno number;
v_COMPNAME varchar2(20);
v_points number;
BEGIN
select :old.SID,:old.TERM,:old.SECTNO into v_sid,v_term,v_sectno from enrolls;
select COMPNAME,points into v_compname,v_points from scores
where scores.sid=v_sid and scores.term=v_term and scores.sectno=v_sectno;
INSERT into DELETED_SCORES (SID,TERM,SECTNO,compname,points)
values (v_sid,v_term,v_sectno,v_compname,v_points);
DELETE FROM SCORES
WHERE SID=V_SID AND TERM=V_TERM AND SECTNO=V_SECTNO;
END;
/
There are two table, which is enrolls and scores. And SCORES table has a combined foreign keys including SID,TERM,AND SECTNO referring to table ENROLLS.
The trigger is now successfully compiled, but there is a problem shown as below:
Error starting at line : 24 in command -
DELETE FROM enrolls
WHERE SID=1111 and term='F12' and sectno=1031
Error report -
SQL Error: ORA-04091: table C16_HE_JIEL.ENROLLS is mutating, trigger/function may not see it
ORA-06512: at "C16_HE_JIEL.HW3", line 8
ORA-04088: error during execution of trigger 'C16_HE_JIEL.HW3'
04091. 00000 - "table %s.%s is mutating, trigger/function may not see it"
*Cause: A trigger (or a user defined plsql function that is referenced in
this statement) attempted to look at (or modify) a table that was
in the middle of being modified by the statement which fired it.
*Action: Rewrite the trigger (or function) so it does not read that table.

Answer 1

never use before trigger for DML. In some situations before trigger can be fired more than once for the same row. It might happen in situations when your session is blocked by other session updating/deleting the same row.
Use compound trigger and apply DML changes in after trigger session.
Then you can be sure, that data you see are really correct.
The mutating table error means that Oracle can not quarantee that what you're doing is deterministic.

Answer 2

This select:
select :old.SID,:old.TERM,:old.SECTNO into v_sid,v_term,v_sectno from enrolls;
Is invalid for two reasons:
you can't select from the table where the trigger fired on
you are selecting all rows from that table, not just one.
The select isn't needed anyway, you can simply use the values in the OLD record directly (and remove the invalid select ... from enrolls)
select COMPNAME,points
into v_compname,v_points
from scores
where scores.sid = :old.SID
and scores.term = :old.TERM
and scores.sectno = :old.SECTNO;
The above statement requires that the combination of (sid, term, secno) is unique in the table scores. If that is not the case and you need to insert multiple rows with into deleted_scores you need to use an INSERT based on a select.
Which then removes the need for variables completely. So the whole trigger can be simplified to:
CREATE OR REPLACE TRIGGER hw3
BEFORE DELETE on ENROLLS
for EACH ROW
BEGIN
INSERT into DELETED_SCORES (SID,TERM,SECTNO,compname,points)
select sid, term, sectno, compname, points
from scores
where scores.sid = :old.SID
and scores.term = :old.TERM
and scores.sectno = :old.SECTNO;
DELETE FROM SCORES
WHERE SID = :OLD.sid
AND TERM = :OLD.term
AND SECTNO = :OLD.sectno;
END;
/
More details about the "table is mutating" restriction can be found in the manual:
http://docs.oracle.com/cd/E11882_01/appdev.112/e25519/triggers.htm#LNPLS759