case 用在 UPDATE

    UPDATE [Table1]
        SET [StatuID] = CASE WHEN  [StatuID] - 1 < 0 THEN 0 ELSE [StatuID] - 1 END
        WHERE [IndexId] = @IndexId;

IsExcellent = CASE ISNULL(ex.MmId,0) WHEN 0 THEN 0 ELSE 1 END