Trigger with subquery - oracle

Question

I'm trying to create a trigger that controls cycles in a self-referencing table.
Unfortunately I've got an error. Could anyone tell me what I'm doing wrong?
CREATE OR REPLACE TRIGGER CHECK_CYCLE
BEFORE INSERT OR UPDATE ON DEPARTMENTS
FOR EACH ROW
BEGIN
IF :NEW.PARENT_ID =
(SELECT ID, PARENT_ID, NAME, LEVEL,
CONNECT_BY_ISLEAF AS ISLEAF,
PRIOR NAME AS PARENT_NAME,
CONNECT_BY_ROOT NAME AS ROOT
FROM DEPARTMENTS
START WITH PARENT_ID IS NULL
CONNECT BY PRIOR ID = PARENT_ID)
THEN
RAISE_APPLICATION_ERROR(20000, 'Sorry.');
END IF;
END;
Error(9,25): PLS-00103: Encountered the symbol "AS" when expecting one of the following:
, from

Answer 1

This seems to be the parser getting confused. If you remove the AS ROOT completely then it reverts to the expected PLS-00405: subquery not allowed in this context error. No idea why it is confused - you can run that query standalone - but since you can't do what you're attempting it's probably not worth worrying about too much.
Your subquery also has three columns and will return multiple rows, so comparing with the current row's scalar PARENT_ID isn't going to work. You could run your subquery separately and select the value you're actually interested in into a local variable, and check against that.
But you have a before-insert trigger and you're querying the table the trigger is against, so you'll get a mutating table exception anyway when you try to insert (or at least, if you try to insert multiple rows at once).
If I understand what you're trying to achieve, you can use an after-insert trigger instead:
create or replace trigger check_cycle
after insert or update on departments
declare
l_hascycle pls_integer;
begin
select max(connect_by_iscycle)
into l_hascycle
from departments
start with parent_id is null
connect by nocycle prior id = parent_id;
if l_hascycle = 1 then
raise_application_error(-20000, 'Sorry.');
end if;
end;
/
This uses the CONNECT_BY_ISCYCLE pseudocolumn, which will be zero for all rows if there is no cycling, and one for any column that cycles. Selecting and checking the max() of that means you'll have a single value of 0 or 1 in the local l_hascycle variable, and you can use that to decide whether to throw an exception.
Inserting a couple of test rows on top of existing non-cycling data such as:
ID PARENT_ID NAME
---------- ---------- ------------
1 Test
2 1 Test
... where the first new row is OK and the second would cause a cycle:
insert into departments (id, parent_id, name) values (3, 2, 'OK');
1 row inserted.
insert into departments (id, parent_id, name) values (2, 3, 'Cycles');
ORA-20000: Sorry.
ORA-06512: at "SCHEMA.CHECK_CYCLE", line 11
ORA-04088: error during execution of trigger 'SCHEMA.CHECK_CYCLE'
The first insert is still in effect (but not yet committed), the second was implicitly rolled back:
select * from departments:
ID PARENT_ID NAME
---------- ---------- ------------
1 Test
2 1 Test
3 2 OK
This won't catch cycles in data that is not connected to an existing tree culminating in a null parent, because of the start with clause; so for exampel with data you could insert 4,5 and 5,4 without raising an exception, as there is no route from either 4 or 5 to a row with a null parent. But that's a different issue and something you might want to test for separately.
It also can't see data from uncommitted changes in other sessions, so it's possible for two sessions to simultaneously insert rows that are valid independently but which still would form a cycle once both are committed.

Answer 2

I think I found the error in your query, it's in this line -
CONNECT_BY_ROOT NAME AS ROOT
which is not built correctly.
Try changing it to something like
CONNECT_BY_ROOT AS ROOT