Update Triggers...how to figure out which columns have changed

[Richard Foster]

Let's assume you have a table, X, with columns A, B, and C. If you create an update trigger on X, how do you determine which one of the columns has changed? And, don't say that you can compare the OLD value to the NEW value as that doesn't work. If the update statement is the following, update X set B = 'house' where A = 1, the update trigger returns NULL for the value of NEW.C. So, if I were to compare NEW.C to OLD.C, I would conclude that an update on C had occurred because 'house' <> NULL. But, no update on column C occurred. So, how do I detect which columns have been updated?

Thanks.

- Rich