Use a Trigger After Insert for updating a table? - sql

Question

I'm using a Trigger on SQL server to update a table stock when a sell is inserted into another table, but the trigger is not doing anything to the table, I suspect I must have an error I can't decipher. When I execute the test Inserts it shows no change to the first table.
The tables are:
Sku VARCHAR (50) PRIMARY KEY,
Stock NUMERIC (38)
);
CREATE TABLE dbo.Salida_Producto (
Numero_Salida INT PRIMARY KEY,
Sku VARCHAR (50),
Cantidad_Salida INT,
FOREIGN KEY(Sku) REFERENCES Stock(Sku)
);
--Test Tabla Stock. Test Values.
INSERT INTO dbo.Stock VALUES ('El Mitchies',100);
INSERT INTO dbo.Stock VALUES ('La Karencilla',200);
INSERT INTO dbo.Stock VALUES ('Perro',300);```
The Trigger:
CREATE TRIGGER [dbo].[tr_for_insert]
ON [dbo].[Salida_Producto]
AFTER INSERT
AS
BEGIN
DECLARE
#Item varchar,
#Cuantos numeric
SELECT #Item = INSERTED.Sku,
#Cuantos = INSERTED.Cantidad_Salida
FROM INSERTED
UPDATE Stock
SET Stock = Stock - #Cuantos
WHERE Sku = #Item
END;
GO
Test Inserts
INSERT INTO dbo.Salida_Producto VALUES (1, 'El Mitchies',3);
INSERT INTO dbo.Salida_Producto VALUES (2, 'La Karencilla',6);
INSERT INTO dbo.Salida_Producto VALUES (3,'Perro',130); ```
The problem I have is that the message box says:
(0 row(s) affected)
(1 row(s) affected)

Answer 1

You have thought your trigger with a row by row approach. It is not good practice. You must learn to think with a "by set" approach.
So a better way to achieve your goal should be something like :
Update S
Set S.Stock = S.Stock - I.Cantidad_Salida
From Stock S
Inner Join inserted I
On S.Sku = I.Sku