You should be able to access the INSERTED
table and retrieve ID or table's primary key. Something similar to this example ...
CREATE TRIGGER [dbo].[after_update] ON [dbo].[MYTABLE]
AFTER UPDATE AS
BEGIN
DECLARE @id AS INT
SELECT @id = [IdColumnName]
FROM INSERTED
UPDATE MYTABLE
SET mytable.CHANGED_ON = GETDATE(),
CHANGED_BY=USER_NAME(USER_ID())
WHERE [IdColumnName] = @id
Here's a link on MSDN on the INSERTED
and DELETED
tables available when using triggers: http://msdn.microsoft.com/en-au/library/ms191300.aspx