oralce case when then end 用法

select (case
         when T_TEACHER_TRAIN_INFO.Qualified = '1' then
          '合格'
         when T_TEACHER_TRAIN_INFO.Qualified = '2' then
          '不合格'
         when T_TEACHER_TRAIN_INFO.Qualified = '合格' then
          '合格'
         when T_TEACHER_TRAIN_INFO.Qualified = '不合格' then
          '不合格'
       end) as Qualified
  from T_TEACHER_TRAIN_INFO