Update a Table Based on Data from Another Table

This first update is supplying data to TableA from TableB if either field is missing.

Update TableA a
   Set (
         a.Field1
       , a.Field2
       ) = (
            Select b.Field1
                 , b.Field2
              From TableB b
             Where b.Key = a.Key
           )
Where Field1 Is Null
   Or Field2 Is Null;

The second example updates TableA with the data from TableB. The 'Where Exists' is necessary if TableA has records with keys that does not find a match on TableB. Without it, the data in TableA would be set to null when a key can't be found in TableB.

Update TableA a
   Set (
         a.Field1
       , a.Field2
       ) = (
            Select b.Field1
                 , b.Field2
              From TableB b
              Where b.Key = a.Key
           )
Where Exists (
              Select 1
                From TableB b
               Where b.Key = a.Key
             );

For examples and more details, see an article written by Duke Ganote at http://tinyurl.com/2vphxg.