How to optimize a trigger? - sql

Question

CREATE TRIGGER T
ON TABLE_2
AFTER INSERT
AS
DECLARE #bought_t int,
#name_t varchar(20)
SELECT #name_t = name_t
FROM inserted
SELECT #bought_t = bought_t
FROM TABLE_1
WHERE name_t = #name_t
IF #bought_t < 100
BEGIN
UPDATE TABLE_1
SET bought_t = #bought_t + 1
WHERE TABLE_1.name_t = #name_t
END
ELSE
ROLLBACK TRANSACTION
The column (TABLE_1) I'm making the update to after the insert happens in the 'TABLE_2' is supposed to hold values between 50 and 100. So I'm asking If this trigger is as professional and optimized as It could be? or I have some flaws that could lead to bugs/security issues.

Answer 1

Basically, you need to completely rewrite your trigger to be set-based and to be able to work with multiple rows in the Inserted pseudo table.
Fortunately, that also makes it easier - in my opinion - try something like this:
CREATE TRIGGER T
ON TABLE_2
AFTER INSERT
AS
UPDATE T1
SET bought_t = bought_t + 1
FROM TABLE_1 T1
INNER JOIN Inserted i ON i.name_t = T1.name_t
WHERE T1.bought_t < 100
UPDATE: demo to prove this works:
-- create the two tables
CREATE TABLE TABLE_2 (ID INT NOT NULL IDENTITY(1,1), ProdName VARCHAR(50))
CREATE TABLE TABLE_1 (ProdName VARCHAR(50), Bought INT)
GO
-- create trigger on "TABLE_2" to update "TABLE_1"
CREATE TRIGGER T2Insert
ON TABLE_2
AFTER INSERT
AS
UPDATE T1
SET Bought = Bought + 1
FROM TABLE_1 T1
INNER JOIN Inserted i ON T1.ProdName = i.ProdName
WHERE T1.Bought < 100
GO
-- initialize TABLE_1 with some seed data
INSERT INTO dbo.TABLE_1 (ProdName, Bought)
VALUES ( 'Prod1', 0), ('Prod2', 20), ('Prod3', 40), ('Prod4', 40), ('Prod100', 100)
-- insert new values into TABLE_2
INSERT INTO dbo.TABLE_2 (ProdName)
VALUES ('Prod1'), ('Prod100'), ('Prod2'), ('Prod4')
-- get data to check
SELECT * FROM dbo.TABLE_1
This renders output:
As you can easily see:
Prod1, Prod2, Prod4 that were inserted caused an update of the value Bought
Prod100 which was also inserted did not cause an update of Bought
UPDATE #2: if you need to be able to insert multiple identical values at once, you need to slightly enhance your trigger like this:
CREATE TRIGGER T2Insert
ON TABLE_2
AFTER INSERT
AS
-- declare table variable to hold names and update counts
DECLARE #UpdateCount TABLE (Name VARCHAR(50), UpdCount INT)
-- from the "Inserted" table, determine which names are being
-- inserted how many times using GROUP BY
INSERT INTO #UpdateCount (Name, UpdCount)
SELECT ProdName, COUNT(*)
FROM Inserted
GROUP BY ProdName
-- now join to this temporary table, and update as many times
-- as needed (instead of +1 for all cases)
UPDATE T1
SET Bought = Bought + uc.UpdCount
FROM TABLE_1 T1
INNER JOIN #UpdateCount uc ON uc.Name = T1.ProdName
WHERE T1.Bought < 100
GO