SQL Server Triggers and Multiple Rows

A common mistake that I’ve seen is the assumption that a DML trigger is fired once for each record modified.  This is not true.  A good SQL developer will use set-based DML statements to alter multiple records at a time.  Regardless of how many records may have been changed, the table’s DML trigger will fire only once.  The trigger’s INSERTED & DELETED tables will contain all the records affected.  I don’t like triggers, but if you find yourself in a pickle and need one… make sure you code for the possibility that more than one record was changed.

BAD trigger:

CREATE TRIGGER trgTableA ON TableA FOR UPDATE
AS
	DECLARE @ID int, @name varchar(50);

	SELECT @ID = i.ID, @name = i.Name FROM inserted i;

	UPDATE TableB SET Name = @name WHERE ID = @ID;
GO

GOOD trigger:

CREATE TRIGGER trgTableA ON TableA FOR UPDATE
AS
	UPDATE b SET 
		b.Name = i.Name
	FROM TableB b
		INNER JOIN inserted i ON i.ID = b.ID;
GO