The error message says it all. You have more than one row in T2 where the T2.Key value is equal to some T1.Key value. The subquery returns all the rows in T2 where the Key values are equal. Since there is more than one row for some T1.Key, the error is generated because which value of F from the several rows should it use to update that row in T1?
Jeff Mason
Custom Apps, Inc.
www.custom-apps.com