Sql Fiddle这里。

update PhoneNumbers
   set Tel2 = case when Tel1 = Tel2 
                   then null 
                   else Tel2 end,
       Tel3 = case when Tel3 in (Tel1, Tel2) 
                   then null 
                   else Tel3 end,
       Tel4 = case when Tel4 in (Tel1, Tel2, Tel3) 
                   then null 
                   else Tel4 end,
       Tel5 = case when Tel5 in (Tel1, Tel2, Tel3, Tel4) 
                   then null 
                   else Tel5 end,
       Tel6 = case when Tel6 in (Tel1, Tel2, Tel3, Tel4, Tel5) 
                   then null 
                   else Tel6 end