Selecting specific rows from DELETED table within a trigger - sql

Question

I have an intermediate table that defines many-to-many relationship between, for instance, Customer and Orders, like this:
USE [master]
GO
CREATE DATABASE Example
GO
USE [Example]
GO
CREATE TABLE [dbo].[CustomerOrders](
[CustomerId] [int],
[OrderId] [int]
)
GO
INSERT INTO CustomerOrders (CustomerId, OrderId) VALUES (1, 1)
INSERT INTO CustomerOrders (CustomerId, OrderId) VALUES (1, 2)
INSERT INTO CustomerOrders (CustomerId, OrderId) VALUES (2, 1)
GO
CREATE TRIGGER [dbo].[CustomerOrdersRemoved]
ON [dbo].[CustomerOrders]
FOR DELETE
AS
BEGIN
SET NOCOUNT ON;
--IF NOT EXISTS (SELECT CustomerId FROM CustomerOrders
INNER JOIN deleted ON CustomerOrders.CustomerId=deleted.CustomerId)
--this wont work
END
GO
DELETE CustomerOrders WHERE OrderId=1
GO
Now, I need to have an ON DELETE trigger on this table, which would need to update another table based on customers who don't have any orders left in the table. In this case, after the DELETE operation the customer with CustomerId=1 will have 1 order with OrderId=2 left and customer with CustomerId=2 will have no orders left. So I need to get only customer with CustomerId=2 from the deleted vtable within the trigger.
How can I accomplish this?

Answer 1

The problem is that you're using deleted.CustomerId on the right side of a comparison.
I was doing something similar to what you're doing.
and the solution is to define a #variable to hold deleted.CustomerId (After your SET NO COUNT line), then compare CustomerOrders.CustomerId to the variable.
EDIT:
Here's the code which works just fine, It is a trigger to update movie rating by the average rating it got from tblRating(MovieID, UserID, rating) which also represents a many-to-many relationship between tblMovies and tblUsers, this one uses inserted but it is the same with deleted:
CREATE TRIGGER trUpdateRating
ON [dbo].[tblRating]
AFTER INSERT
AS
BEGIN
SET NOCOUNT ON
DECLARE #rate float, #mid int
SET #rate = 0;
-- here is the assignment I was talking about and it is valid
SELECT #mid=MovieID from inserted;
SELECT #rate=AVG(isnull(Rating, 0)) FROM tblRating WHERE MovieID=#mid;
-- and here is the comparison
UPDATE tblMovies
SET avg_rating = #rate
WHERE ID=#mid;
END

Answer 2

My hands are faster than my head :-/
The answer is
SELECT CustomerId FROM deleted WHERE CustomerId NOT IN (SELECT CustomerId FROM CustomerOrders)