I'm tearing my hair out with this. It should be pretty simple, I think, but either it's a lot more complicated than I think or I've just been staring at it too long and I'm not thinking right. I want to update a table based on a related value from the same table. (Long story short it's a very bad db design that I'm trying to fix as best I can.) Here's the query I'm running (in SQL Server): UPDATE Reservations SET ReservedBy = r2.ReservedBy FROM Reservations r, Reservations r2 WHERE r.ReservedBy IS NULL AND r.ParentReservationID = r2.ReservationID I get this error: Server: Msg 8154, Level 16, State 1, Line 1 The table 'Reservations' is ambiguous. I've tried several variations on this query, as well as using a correlated subquery. And much googling. No luck. Ideas?