SQL Server INSERT in an AFTER UPDATE Trigger - sql-server

Question

I'm new to SQL Server, and I'm trying to build a simple update trigger that writes a row to a staging table whenever the column ceu_amount is updated from zero to any number greater than zero.
From using PRINT statements, I know that the variables are containing the correct values to execute the INSERT statement, but no rows are being inserted.
Can you help?
CREATE TRIGGER [dbo].[TRG_Product_Function_Modified] ON [dbo].[Product_Function]
AFTER UPDATE
AS
BEGIN
SET NOCOUNT ON;
--
-- Variable definitions
--
DECLARE #product_code_new as varchar(31)
DECLARE #product_code_old as varchar(31)
--
-- Check if the staging table needs to be updated.
--
SELECT #product_code_new = product_code FROM Inserted where ISNULL(ceu_amount,0) > 0;
SELECT #product_code_old = product_code FROM Deleted where ISNULL(ceu_amount,0) = 0;
IF #product_code_new IS NOT NULL
AND #product_code_old IS NOT NULL
INSERT INTO Product_Function_Staging VALUES (#product_code_new,CURRENT_TIMESTAMP);
END;

Answer 1

This part of code looks suspicious to me..
SELECT #product_code_new = product_code FROM Inserted where ISNULL(ceu_amount,0) > 0;
SELECT #product_code_old = product_code FROM Deleted where ISNULL(ceu_amount,0) = 0;
IF #product_code_new IS NOT NULL
AND #product_code_old IS NOT NULL
INSERT INTO Product_Function_Staging VALUES (#product_code_new,CURRENT_TIMESTAMP);
The above will work fine ,if there is only one row updated,what if there is more than one value..the product_code will default to last value
You can change the above part of code to below
Insert into Product_Function_Staging
select product_code ,CURRENT_TIMESTAMP from inserted where product_code is not null

Answer 2

You will get undetermined values for #product_code_new if there are more than one rows updated with ceu_amount>0; Similar for #product_code_old if more than one rows updated with ceu_amount NULL or equal 0.
Can you post some sample data?

Answer 3

I would not use variables like that in a trigger, since what causes the trigger could be an update to more than one row, at which point you would have multiple rows in your updated and deleted tables.
I think we can more safely and efficiently make this insert with one simple query, though I'm assuming you have a unique key to use:
CREATE TRIGGER [dbo].[TRG_Product_Function_Modified] ON [dbo].[Product_Function]
AFTER UPDATE
AS
BEGIN
SET NOCOUNT ON;
INSERT INTO Product_Function_Staging
SELECT i.product_code, CURRENT_TIMESTAMP
FROM inserted i
JOIN deleted d ON i.product_code = d.product_code -- assuming product_code is unique
WHERE i.ceu_amount > 0 -- new value > 0
AND ISNULL(d.ceu_amount, 0) = 0; -- old value null or 0
END;
I'm not sure where you need to check for nulls in your data, so I've made a best guess in the where clause.

Answer 4

Try using this
CREATE TRIGGER [dbo].[Customer_UPDATE]
ON [dbo].[Customers]
AFTER UPDATE
AS
BEGIN
SET NOCOUNT ON;
DECLARE #CustomerId INT
DECLARE #Action VARCHAR(50)
SELECT #CustomerId = INSERTED.CustomerId
FROM INSERTED
IF UPDATE(Name)
BEGIN
SET #Action = 'Updated Name'
END
IF UPDATE(Country)
BEGIN
SET #Action = 'Updated Country'
END
INSERT INTO CustomerLogs
VALUES(#CustomerId, #Action)
END