case when then else end

示例一

SELECT 学号, 姓名, 
 等级= 
    CASE 
    WHEN 总学分 IS NULL THEN ‘尚未选课’
       WHEN 总学分 < 50 THEN ‘不及格’
        WHEN 总学分 >=50 and 总学分<=52 THEN ‘合格’
        ELSE ‘优秀’
    END  FROM  XS    WHERE 专业名=’计算机’

 

示例二

update employee
set e_wage =
 case
  when job_level = ’1’ then e_wage*1.08
  when job_level = ’2’ then e_wage*1.07
  when job_level = ’3’ then e_wage*1.06
  else e_wage*1.05 //  或者else  e_wage    
 end

 

示例三

select a, (case a when 1 then '中' else '国' end) AS B  from table1 或者select a, (case a when 1 then '中' else a end) AS B  from table1  但是a的类型必须与'中'的类型一致....因为同一字段不能存放两种不同类型的数据