select * from 社員マスタ where 給与 + iif(手当 is null,0,手当) = (select max(給与 + iif(手当 is null,0,手当)) from 社員マスタ)
select * from 社員マスタ where 給与 + isnull(手当,0) = (select max(給与 + isnull(手当,0)) from 社員マスタ)
select * from 社員マスタ where 給与 + coalesce(手当,0) = (select max(給与 + coalesce(手当,0)) from 社員マスタ)