The code that is included inside an UPDATE trigger runs every time its related table is updated. In most UPDATE triggers, the code in the trigger affects only certain columns, not all of them. Because of this, it is pointless (and a waste of SQL Server resources) to run all of the code in the trigger if the column or columns you are interested in have not been updated. In other words, even if a column you are not interested in is updated, the UPDATE trigger will fire and run its code.
To help reduce the unnecessary running of code in an UPDATE trigger, you can take advantage of one of two different functions: UPDATE() (available in SQL Server 2000/2005) or COLUMNS_UPDATED() (available in SQL Server 7.0 and 2000/2005).
Either function can be used to test to see if a particular column you are interested in has changed or not. Because of this, you can write code in your trigger to only run if the column you are interested in has changed, otherwise you can prevent the code from running if the column you are interested in has not changed. This can reduce the amount of work the trigger needs to do, boosting overall performance of your database.
The UPDATE() function is used to check only one column at a time. The COLUMNS_UPDATED() function can be used to check multiple columns at a time. [7.0, 2000, 2005] Updated 1-10-2005
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