Why does it raise a mutating table error? - sql

Question

I am attenting to create a trigger that before delete a row in the table CLUBS check if the column END_DATE is null, if it's null the row instead of deleting the row it replaces the value with sysdate and if is not null the row doesn't get deleted.
CREATE OR REPLACE TRIGGER TRANSFER_DATA
BEFORE DELETE ON CLUBS
FOR EACH ROW
BEGIN
IF :old.END_DATE IS NULL
THEN UPDATE CLUBS SET END_DATE = SYSDATE;
ELSE
RAISE_APPLICATION_ERROR(-20033, 'No se puede borrar');
END IF;
END;
I've tried this code but it raises an error:
"table %s.%s is mutating, trigger/function may not see it"
How can I make it work?
By the way, I am working on Oracle sqldeveloper

Answer 1

You need to create a view and a INSTEAD OF trigger:
create or replace view v_CLUBS as
SELECT * FROM CLUBS; -- perhaps add 'WHERE END_DATE IS NULL'
CREATE OR REPLACE TRIGGER TRANSFER_DATA
INSTEAD OF DELETE ON V_CLUBS
FOR EACH ROW
BEGIN
IF :old.END_DATE IS NULL THEN
:new.end_date := sysdate;
ELSE
RAISE_APPLICATION_ERROR(-20033, 'No se puede borrar');
END IF;
END;

Answer 2

Processing through a view and an instead of trigger is differently an effective way to go. It seems your prior attempt approached the task incorrectly. A simple view is fully updateble (generally). But in this case you want to intercept the delete and apply the action to the underlying table. Assuming CLUBS is view you process against and CLUBS_TBL is the actual table the trigger you want is:
-- create trigger to process deletes on the view
create or replace trigger del_clubs_isd
instead of delete on clubs
begin
update clubs_tbl
set end_date = sysdate
where id = :old.id;
end del_clubs_isd;
See a full script at fiddle.
Further, as presented the user never needs to know the club is not actually deleted, but just marked to appear that way.