Oracle SQL, avoiding errror "table is mutating" (trigger) - sql

Question

i need trigger which will check if updated worker can be moved to other team
CREATE TABLE WORKERS
(
ID_WORKER NUMBER(4,0), --FK
ID_TEAM NUMBER(2,0) --FK
);
My trigger looks like:
CREATE OR REPLACE TRIGGER TEAM_LIMIT
BEFORE INSERT OR UPDATE OF ID_TEAM ON WORKERS
FOR EACH ROW
DECLARE
V_num NUMBER;
BEGIN
SELECT Count(*) INTO V_num FROM Worker WHERE ID_TEAM=:new.ID_TEAM;
IF V_num >= 5 THEN
RAISE_APPLICATION_ERROR(-20025,' Error nr ... bleble');
END IF;
END;
this generate error : "table %s.%s is mutating, trigger/function may not see it" when row is updated. How to write this statements properly to not generate this kind of error?

Answer 1

You can use a compound trigger, it looks like this (not tested):
CREATE OR REPLACE TRIGGER TEAM_LIMIT
FOR INSERT OR UPDATE OF ID_TEAM ON WORKERS
COMPOUND TRIGGER
V_num NUMBER;
TYPE Row_TableType IS TABLE OF WORKERS.ID_TEAM%TYPE;
AffectedTeams Row_TableType;
BEFORE STATEMENT IS
BEGIN
AffectedTeams := Row_TableType(); -- init the table variable
END BEFORE STATEMENT;
-------------------
BEFORE EACH ROW IS
BEGIN
AffectedTeams.EXTEND;
AffectedTeams(AffectedTeams.LAST) := :NEW.ID_TEAM;
END BEFORE EACH ROW;
--------------------
AFTER STATEMENT IS
BEGIN
FOR i IN AffectedTeams.FIRST..AffectedTeams.LAST LOOP
SELECT Count(*) INTO V_num FROM Worker WHERE ID_TEAM=AffectedTeams(i);
IF V_num >= 5 THEN
RAISE_APPLICATION_ERROR(-20025,' Error nr ... bleble');
END IF;
END LOOP;
END AFTER STATEMENT;
END TEAM_LIMIT;
/

Answer 2

You can use a statement trigger. Statement triggers are only fired once for each statement executed rather than once for each row affected. Statement triggers are useful because they don't have the restriction that they can't query the table on which the trigger is declared. They don't have access to the :OLD and :NEW row values but with a little thought you can still accomplish what you're trying to do:
CREATE OR REPLACE TRIGGER TEAM_LIMIT
BEFORE INSERT OR UPDATE OF ID_TEAM ON WORKERS
-- Note: no FOR EACH ROW - therefore, this is a statement trigger
DECLARE
nMax_team_count NUMBER;
BEGIN
SELECT MAX(TEAM_COUNT)
INTO nMax_team_count
FROM (SELECT ID_TEAM, COUNT(*) AS TEAM_COUNT
FROM WORKERS
GROUP BY ID_TEAM));
IF nMax_team_count >= 5 THEN
RAISE_APPLICATION_ERROR(-20025,' Error nr ... bleble');
END IF;
END TEAM_LIMIT;
Instead of looking at the team of the particular work which has been updated we find the counts of workers on each team, then extract the largest count, and if it's more than four we raise the appropriate exception.
Share and enjoy.