I'm wondering if it's possible to use UPDATE on a row where it's unknown whether the column being updated already has a value or not.

If the column's value is set to NULL, will UPDATE work? Or do I really need to test for NULL and write either an UPDATE or an INSERT query? I'd really like to streamline my code by not having to do the test, but I don't know how far UPDATE will go.

By the way, I looked at REPLACE INTO, but I'm not worried about an entire row of data, just a single column in an existing row.

Thanks,
Glenn