Trying to delete a row based upon condition defined in my trigger (SQL) - oracle

Question

I am trying to create a row level trigger to delete a row if a value in the row is being made NULL. My business parameters state that if a value is being made null, then the row must be deleted. Also, I cannot use a global variable.
BEGIN
IF :NEW.EXHIBIT_ID IS NULL THEN
DELETE SHOWING
WHERE EXHIBIT_ID = :OLD.EXHIBIT_ID;
END IF;
I get the following errors:
ORA-04091: table ISA722.SHOWING is mutating, trigger/function may not see it
ORA-06512: at "ISA722.TRG_EXPAINT", line 7
ORA-04088: error during execution of trigger 'ISA722.TRG_EXPAINT'
When executing this query:
UPDATE SHOWING
SET EXHIBIT_ID = NULL
WHERE PAINT_ID = 5104

Answer 1

As already indicated this is a terrible idea/design. Triggers are very poor methods for enforcing business rules. These should be enforced in the application or better (IMO) by a stored procedure called by the application. In this case not only is it a bad idea, but it cannot be implemented as desired. Within a trigger Oracle does not permit accessing the table the trigger fired was fired on. That is what mutating indicates. Think of trying to debug this or resolve a problem a week later. Nevertheless this non-sense can be accomplished by creating view and processing against it instead of the table.
-- setup
create table showing (exhibit_id integer, exhibit_name varchar2(50));
create view show as select * from showing;
-- trigger on VIEW
create or replace trigger show_iiur
instead of insert or update on show
for each row
begin
merge into showing
using (select :new.exhibit_id new_eid
, :old.exhibit_id old_eid
, :new.exhibit_name new_ename
from dual
) on (exhibit_id = old_eid)
when matched then
update set exhibit_name = new_ename
delete where new_eid is null
when not matched then
insert (exhibit_id, exhibit_name)
values (:new.exhibit_id, :new.exhibit_name);
end ;
-- test data
insert into show(exhibit_id, exhibit_name)
select 1,'abc' from dual union all
select 2,'def' from dual union all
select 3,'ghi' from dual;
-- 3 rows inserted
select * from show;
--- test
update show
set exhibit_name = 'XyZ'
where exhibit_id = 3;
-- 1 row updated
-- Now for the requested action. Turn the UPDATE into a DELETE
update show
set exhibit_id = null
where exhibit_name = 'def';
-- 1 row updated
select * from show;
-- table and view are the same (expect o rows)
select * from show MINUS select * from showing
UNION ALL
select * from showing MINUS select * from show;
Again this is a bad option yet you can do. But just because you can doesn't mean you should. Or that you'll be happy with the result. Good Luck.

Answer 2

You have written a trigger that fires after or before a row change. This is in the middle of an execution. You cannot delete a row from the same table in that moment.
So you must write an after statement trigger instead that only fires when the whole statement has run.
create or replace trigger mytrigger
after update of exhibit_id on showing
begin
delete from showing where exhibit_id is null;
end mytrigger;
Demo: https://dbfiddle.uk/?rdbms=oracle_18&fiddle=dd5ade700d49daf14f4cdc71aed48e17

Answer 3

What you can do is create an extra column like is_to_be_deleted in the same table, and do this:
UPDATE SHOWING
SET EXHIBIT_ID = NULL, is_to_be_deleted = 'Y'
WHERE PAINT_ID = 5104;
You can use this parameter to implement your business logic of not showing the null details.
And later you can schedule a batch delete on that table to clean up these rows (or maybe archive it).
Benefit: you can avoid an extra unnecessary trigger on that table.
Nobody, will suggest you to use trigger to do this type of delete as it is expensive.