MySQL ：max GROUP 部门最高 EXPLAIN 调试

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