SQL Server : update trigger seeming to affect wrong column - sql-server

Question

Thanks for looking. I'm trying to write a SQL Server trigger that when a new record is added containing date information, will add the day of the week to the DayOfWeek column. Here's my table, with the columns in order:
Food table:
FoodName **varchar(20)**
CategoryID (FK) **int**
Price **smallmoney**
StoreID (FK) **int**
Date **datetime**
DayOfWeek **varchar(9)**
ShopperID (FK) **int**
Week **int**
Here is the trigger I've written:
-- Create a trigger to update day of the week when a record is inserted
CREATE TRIGGER DOW
ON Food
FOR INSERT
AS
BEGIN
-- Declare a variable to hold the date ID
DECLARE #dateID DATETIME
-- Get the date from the new record and store it in #dateID
SELECT #dateID = Date FROM Food
-- Insert day of the week based on the inserted date
INSERT INTO Food (DayOfWeek)
SELECT DATENAME(dw, #dateID)
END
GO
SQL Server seemed to accept the procedure, but when I ran another procedure to insert a new record, I got this error:
Msg 515, Level 16, State 2, Procedure DOW, Line 8 [Batch Start Line 21]
Cannot insert the value NULL into column 'Week', table *******; column does not allow nulls. INSERT fails.
I am not sure why this trigger is affecting the 'Week' column at all. The code should take the value entered for the Date and use the DATENAME(dw,...) function to return the day of the week, which should go into the DayOfWeek column. I've written a stored procedure that accepts a date as input and inserts the corresponding day of the week into the record, and it works just fine, but this trigger doesn't seem to want to cooperate. I'm stumped!

Answer 1

What your trigger does:
it fetches a Date from your table (the last one that is returned) which is not necessarily the last inserted value.
it tries to insert a new record with just the DayOfWeek of that Date specified.
it fails, because at least the Week must also be specified.
I guess that you want to update the value of the DayOfWeek for the inserted row(s) instead. To be able to do so, there must be a way to identify the row(s) that need to be updated in the Food table by knowing the values of the inserted rows. To be sure to update the correct rows, there should be a primary key that allows you to identify them. For sure you have such a primary key, and I guess that it's named FoodID, so probably you wanted to do this:
CREATE TRIGGER DOW ON Food
FOR INSERT
AS
BEGIN
SET NOCOUNT ON;
-- update the day of the week for the inserted rows
UPDATE Food
SET [DayOfWeek] = DATENAME(dw, f.[Date])
FROM Food f
INNER JOIN inserted i ON f.FoodID = i.FoodID
END
GO

Answer 2

There are some major problems with your trigger. In triggers, there is an inserted table (on inserts and updates) and deleted table (on deletes and updates). You should be using this table's information to know what records need updated.
This is bad because a trigger can have multiple rows
This SQL simply will not work correctly if you insert multiple rows.
DECLARE #dateID DATETIME
SELECT #dateID = Date FROM Food
This SQL is trying to insert a new row which is causing your NULL error
It is not trying to update the row you are inserting
INSERT INTO Food (DayOfWeek)
SELECT DATENAME(dw, #dateID)
It would need to be an INSTEAD OF trigger to avoid the null constraint on the column. Wolfgang's answer will still cause a null constraint error, because after triggers run AFTER the data is inserted. An INSTEAD OF trigger will run in place of the the actual insert.
CREATE TRIGGER DOW ON Food
INSTEAD OF INSERT
AS
BEGIN
SET NOCOUNT ON;
-- update the day of the week for the inserted rows
INSERT INTO Food (FoodName,CategoryID,Price,StoreID,[Date],ShopperID,[Week],[DayOfWeek])
SELECT
FoodName,CategoryID,Price,StoreID,[Date],ShopperID,[Week],DATENAME(dw, [Date]) AS [DayOfWeek]
FROM inserted
END
GO
Personally, I think storing the week and day of week is a bad idea. You already have a value that can derive that information (Date). Any time you have multiple columns that are essentially duplicate data, you will run into maintenance pain.