Update real_table
         Set columns = TT.columns
From
         real_table RT Join
         temp_table TT On
                RT.unique_key = TT.unique_key

Insert Into real_table
         Select
               columns
         From
               temp_table TT
         Where 
               Not Exists(Select 1 from real_table RT where TT.unique_key = RT.unique_key)