SELECT id,stuid FROM Stu GROUP BY stuid SELECT MAX(id),stuid FROM stuGROUP BY stuid

如果有多条数据 ，按第一条sql 执行 GROUP by 数据，拿到的可能会是这个学生的旧数据

而第二条加上max，则可以拿到他最新的数据

 

每个部门工资最高的员工

select A.* from tab_salary A inner join (select max(salary) as salary,departid as departid from tab_depart group by departid) B on A.salary=B.salary and A.departid=B.departid

sql 查出某部门男女分别最高薪资

1，版本

select f.* from money f INNER JOIN (select max(moneys) AS moneys,sex,`name` as departid from money group by sex) B on f.moneys=B.moneys and f.sex=B.sex WHERE f.dept=1

改进：

select f.* from money f INNER JOIN (select max(mn.moneys) AS moneys,mn.sex from money mn group by mn.sex) B on f.moneys=B.moneys and f.sex=B.sex WHERE f.dept=1

 

调试sql

EXPLAIN  select