trigger - select count into - ORA-04091 - oracle

Question

I want to INSERT or UPDATE on table (CMS_CONTENT_ENROLLMENT) and then do a count over the same table and update the result into another table. Sadly I'm getting a ORA-04091: table name is mutating, trigger/function may not see it
CREATE OR REPLACE TRIGGER CMS_CONTENT_ENROLLMENT_CNT_TR
AFTER INSERT OR UPDATE
ON CMS_CONTENT_ENROLLMENT
FOR EACH ROW
DECLARE
BEGIN
UPDATE CMS_CONTENT CNT
SET CNT.REGISTRATIONCOUNT =
(
SELECT COUNT (ENROLLMENTID)
FROM CMS_CONTENT_ENROLLMENT
WHERE DELETED = 0 AND CONTENTID = CNT.CONTENTID
)
WHERE CNT.CONTENTID = :NEW.CONTENTID;
END;
/

Answer 1

Words of warning: I don't think it's a good idea to embed complex application logic code in triggers because the code is spread among many objects and it makes it hard to test and maintain. Usually the ORA-04091 is an indication that you are trying to perform something too complex for triggers. This can be circumvented but will result in complex logic often flawed in some ways, especially in a multi-user environment like a database. For this reason if you want a summary table I would advice a regular view, a materialized view or procedural logic.
If you want to go with triggers anyway, you could make it work with incremental logic, like this (untested, assumes that you never update contendid, that deleted is NOT NULL, that you never actually delete from the table and that the row exists in the summary table):
CREATE OR REPLACE TRIGGER CMS_CONTENT_ENROLLMENT_CNT_TR
AFTER INSERT OR UPDATE ON CMS_CONTENT_ENROLLMENT
FOR EACH ROW
DECLARE
BEGIN
IF :NEW.deleted = 0 AND ((inserting) OR :OLD.deleted != 0) THEN
UPDATE CMS_CONTENT CNT
SET CNT.REGISTRATIONCOUNT = CNT.REGISTRATIONCOUNT + 1
WHERE cnt.contentid = :NEW.contentid;
ELSIF :NEW.deleted != 0 AND :OLD.deleted = 0 THEN
UPDATE CMS_CONTENT CNT
SET CNT.REGISTRATIONCOUNT = CNT.REGISTRATIONCOUNT - 1
WHERE cnt.contentid = :NEW.contentid;
END IF;
END;
/

Answer 2

Your FOR EACH ROW trigger is querying the same table that it inserts/updates. Since the trigger fires for each row, the result of the query (if it were allowed) would be indeterminate depending on the order in which the rows are processed - this is illogical, so Oracle raises ORA-04091: table name is mutating, trigger/function may not see it.
In addition, I strongly echo Vincent's warning about writing trigger-based logic in a multi-user (or even multi-session) environment. Oracle will allow multiple versions of your code to run simultaneously, and they will not see each other's changes until they commit. Your trigger will have already "counted" the rows that it could see at the time, which potentially will mean both sessions will get it wrong.
The only way around this is (a) architect your application so that it doesn't need to store the count anywhere, instead query it on the fly when needed; or (b) introduce some form of serialization to stop multiple processes trying to do inserts/updates on the table at the same time.
I strongly recommend option (a). Option (b) is very easy to get wrong unless you first gain a very good understanding of how Oracle works.