SQL集合函数中利用case when then 技巧

我们都知道SQL中适用case when then来转化数据库中的信息

比如  select (case sex when 0 then ‘男‘ else ‘女‘ end) AS sex  from studentInfo

那么在集合函数中它有什么用呢 ？

SELECT sex FROM student

SELECT sex,COUNT(*) AS num FROM student GROUP BY sex

SELECT sex,province,COUNT(*) AS num FROM student GROUP BY sex,province
SELECT sex,province,COUNT(province) AS num FROM student GROUP BY sex,province

SELECT sex,province,COUNT(CASE province WHEN ‘浙江省‘ THEN ‘浙江省‘ ELSE province END ) AS num FROM student GROUP BY sex

SELECT sex,COUNT(CASE province WHEN ‘江西省‘ THEN ‘江西省‘ END) AS 江西省  ,COUNT(CASE province WHEN ‘广东省‘ THEN ‘广东省‘ END) AS 广东省,
COUNT(CASE province WHEN ‘浙江省‘ THEN ‘浙江省‘ END) AS 浙江省 FROM student GROUP BY sex