Why won't my master table column increment in my trigger? - sql

Question

I need to create a trigger that will keep track of the number of times a movie is rented from a business like Blockbuster. I need a separate trigger for an insert, a delete and an update.
The column that tracks this number of times rented is called num_rentals and has a datatype of int. This column is part of the Movies table which has *movie_id (pk*), movie_title, release_year, movie_description, and num_rentals. The customer_rentals table has item_rental_id(pk), *movie_id(fk*), customer_id.
I searched and found a similar thread here for this and tried using the supplied answer, but to no avail. I executed the following string without any errors but I saw no change in the num_rentals column when I inserted data into either the Movie or the Customer_rentals table. What am I doing wrong?
CREATE TRIGGER dbo.tr_num_rented_insert ON dbo.customer_rentals
FOR INSERT
AS
BEGIN
UPDATE m
SET num_rentals = num_rentals + 1
FROM dbo.movies AS m
INNER JOIN inserted AS i ON m.movie_id = i.movie_id ;
END
I added the num_rentals field to the table later and I also need to know how to initialize the field value to zero for all records currently in the Movies table.
I want to understand this as much as I want the answer so assistance is greatly appreciated. I read that there may be more efficient ways to manage this type of data, but this is the way my instructor wants it. Thank you in advance!

Answer 1

I suspect your problem is null num_rental column. So you're adding null to 1, resulting in null.
Personally, I'd set the num_rentals column to non-nullable with a default of zero, but assuming you can't do that, use isnull().
Like so
alter TRIGGER dbo.tr_num_rented_insert ON dbo.customer_rentals
FOR INSERT
AS
BEGIN
UPDATE m
SET num_rentals = isnull(num_rentals,0) + 1
FROM dbo.movies AS m
INNER JOIN inserted AS i ON m.movie_id = i.movie_id ;
END
However, even though this works, there is a problem with it; if you add multiple rows to customer_rentals, it's only going to increment by one. Personally, I'd change the trigger to account for that.
Like this
alter TRIGGER dbo.tr_num_rented_insert ON dbo.customer_rentals
FOR INSERT
AS
BEGIN
UPDATE m
SET num_rentals = isnull(num_rentals,0) + (select COUNT(*) from inserted where movie_id = m.movie_id)
FROM dbo.movies AS m
where movie_id in (select movie_id from inserted)
END
As far as the duplicate information goes, Aaron is right that this is needlessly redundant and in my experience, this type of thing often gets out of sync. With such a simple database, the num_rentals column is overkill (to be generous), but your movie database is a contrived example to teach you a concept. Basically, sometimes you will want calculated values to be easily accessible or filtered on. Take SO rep for example, I assume they don't recalc that every time they display it.

Answer 2

Aaron's frequently upvoted comment suggests that you are doing something unwise. However, since you are doing what your instructor is specifying, I suggest that you look at a couple of things.
First, will your update query work for the initial rental? If the initial value is 0 it will. If the initial value is null it won't.
Second, you might be referencing the wrong table. The trigger is on customer_rentals but your trigger refers to inserted. That strikes me as odd.
Just so you know, I was one of those who upvoted Aaron's comment.