Table is mutating, trigger/function may not see it ORA-06512 [duplicate] - oracle

Question

This question already has answers here:
Oracle trigger after insert or delete
(3 answers)
Closed 7 years ago.
I have two tables called DetailRental and Video. VID_NUM is the PK of Video and the FK of DetailRental.
What this code wants to achieve is when the Detail_Returndate or Detail_Duedate from DetailRental table changes(update or insert new row), the trigger will check the value of Detail_Returndate row by row. If its value is null, then the corresponding(according to VID_NUM) attribute VID_STATUS from Video table will change to "OUT".
The trigger has been created successfully. However, when I want to update the date. Oracle gives me error:
ORA-04091: table SYSTEM2.DETAILRENTAL is mutating, trigger/function may not see it
ORA-06512: at "SYSTEM2.TRG_VIDEORENTAL_UP", line 3
ORA-04088: error during execution of trigger 'SYSTEM2.TRG_VIDEORENTAL_UP'
1. UPDATE DETAILRENTAL
2. SET DETAIL_RETURNDATE = null
3. WHERE RENT_NUM = 1006 AND VID_NUM = 61367
Below is my code:
CREATE OR REPLACE TRIGGER trg_videorental_up
AFTER INSERT OR UPDATE OF DETAIL_RETURNDATE, DETAIL_DUEDATE ON DETAILRENTAL
FOR EACH ROW
AS
DECLARE
DTRD DATE;
BEGIN
SELECT DETAIL_RETURNDATE
INTO DTRD
FROM DETAILRENTAL;
IF DTRD IS NULL
THEN UPDATE VIDEO
SET VIDEO.VID_STATUS = 'OUT'
WHERE EXISTS
(SELECT DETAILRENTAL.VID_NUM
FROM DETAILRENTAL
WHERE DETAILRENTAL.VID_NUM = VIDEO.VID_NUM
);
END IF;
END;
Thank you very much!
problem solved here is the code:
CREATE OR REPLACE TRIGGER trg_videorental_up
AFTER INSERT OR UPDATE OF DETAIL_RETURNDATE, DETAIL_DUEDATE ON DETAILRENTAL
FOR EACH ROW
DECLARE DETAIL_RETURNDATE DATE;
BEGIN
IF :NEW.DETAIL_RETURNDATE IS NULL THEN UPDATE VIDEO SET VID_STATUS = 'OUT' WHERE VID_NUM = :NEW.VID_NUM;
ELSIF :NEW.DETAIL_RETURNDATE > SYSDATE THEN UPDATE VIDEO SET VID_STATUS = 'OUT' WHERE VID_NUM = :NEW.VID_NUM;
ELSIF :NEW.DETAIL_RETURNDATE <= SYSDATE AND TO_CHAR(DETAIL_RETURNDATE)!= '01/01/0001' THEN UPDATE VIDEO SET VID_STATUS = 'IN' WHERE VID_NUM = :NEW.VID_NUM;
ELSIF :NEW.DETAIL_RETURNDATE = '01/01/0001' THEN UPDATE VIDEO SET VID_STATUS = 'LOST' WHERE VID_NUM = :NEW.VID_NUM;
END IF;
END;

Answer 1

A good data model is one in which no redundant information is physically stored. If you can look at one (or more) values in a table.column and figure out what value should be in another table.column, then you've got a redundancy. In your case, a person can see a DETAILRENTAL.DETAIL_DUEDATE for VIDNUM 61367 is not null, and "know" that the VIDEO.STATUS field should be OUT.
Most easily fixed with something like:
1) Create a VIDEO_BASE table, with all VIDEO columns except VID_STATUS:
CREATE TABLE VIDEO_BASE AS
SELECT {list all columns except STATUS}
FROM VIDEO;
2) Drop original VIDEO table and create as a view, VIDEO, which shows all columns of VIDEO_BASE, plus exposes STATUS as a derived field:
CREATE OR REPLACE VIEW VIDEO
AS
SELECT V.*,
CASE WHEN
(
SELECT COUNT(*)
FROM
(
SELECT 'X'
FROM DETAILRENTAL D
WHERE D.VID_NUM = V.VID_NUM
AND DETAIL_RETURNDATE IS NOT NULL
AND ROWNUM <= 1
)
) > 0
THEN 'OUT'
ELSE NULL
END VID_STATUS
FROM VIDEO_BASE V;
In general, if you feel you need a trigger to keep two different tables in sync, you've got a data model problem. In my 15+ years experience with Oracle, the only best way to fix problematic triggers is to fix the data model - the surest way to know that all your triggers are working properly is when the number of triggers in your database is 0.

Answer 2

After reading through #KevinKirkpatrick's answer two or three times, I realise he's right - an individual video's in/out status is derivable from other information in the database. That said, you may have pragmatic reasons for doing it this way.
The bad news is that you can't select from a table within a row trigger on that same table - that's what the "mutating table" problem means. The good news is that in this case you don't really need to.
I don't have an Oracle installation I can test this on, so I make no guarantee of syntactic correctness, but it should be close enough to get you started.
CREATE OR REPLACE TRIGGER trg_videorental_up
AFTER INSERT OR UPDATE
OF detail_duedate, detail_returndate
ON detailrental
FOR EACH ROW
AS
BEGIN
IF :new.detail_returndate IS NULL
AND :new.detail_duedate IS NOT NULL
THEN
UPDATE video
SET status = 'OUT'
WHERE video_num = :new.video_num;
END IF;
END;