Yes it will do the update, but in case you're trying to update the row with row #1 value A from the 2nd table and then row #5 value B from the 2nd table (both linking to the same row in the 1st table) I'm not sure which would hold at the end of the update? I just avoid such situations. Certainly if A=B then you have no real issue. Is there a rule that governs it? I've never thought about/bothered to try an "order by" here but I suppose, if it was permitted, then it would bring some certainly to the update.