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The
Employeetable holds all employees. Every employee has an Id, a salary, and there is also a column for the department Id.+----+-------+--------+--------------+ | Id | Name | Salary | DepartmentId | +----+-------+--------+--------------+ | 1 | Joe | 70000 | 1 | | 2 | Henry | 80000 | 2 | | 3 | Sam | 60000 | 2 | | 4 | Max | 90000 | 1 | +----+-------+--------+--------------+The
Departmenttable holds all departments of the company.+----+----------+ | Id | Name | +----+----------+ | 1 | IT | | 2 | Sales | +----+----------+Write a SQL query to find employees who have the highest salary in each of the departments. For the above tables, Max has the highest salary in the IT department and Henry has the highest salary in the Sales department.
+------------+----------+--------+ | Department | Employee | Salary | +------------+----------+--------+ | IT | Max | 90000 | | Sales | Henry | 80000 | +------------+----------+--------+
先后写了5，6个版本，效率各不相同，挑出典型的5个，来分析一下sql语句的优化
select Department.Name as Department,
Employee.Name as Employee,
Employee.Salary as Salary
from Department join Employee
on Department.Id = Employee.DepartmentId
where (Department.Id, Employee.Salary) in
(select DepartmentId, max(Salary) from Employee group by DepartmentId);
select Department.Name as Department,
Employee.Name as Employee,
Employee.Salary as Salary
from Department join Employee
on Department.Id = Employee.DepartmentId
where (Department.Id, Employee.Salary) in
(select DepartmentId, Salary
from (select * from Employee order by Salary desc) q
group by DepartmentId);
select a.Name as Department,
b.Name as Employee,
b.Salary as Salary
from Department a join Employee b
on a.Id = b.DepartmentId
where exists(select 1 from (select * from Employee order by Salary desc) c
group by DepartmentId
having a.Id = c.DepartmentId and b.Salary = max(c.Salary));
select a.Name as Department,
b.Name as Employee,
b.Salary as Salary
from (Department a join Employee b on a.Id = b.DepartmentId) join
(select c.DepartmentId,max(c.Salary) as Salary from (select * from Employee order by Salary desc) c group by DepartmentId) d
on a.Id = d.DepartmentId and b.Salary = d.Salary;
select a.Name as Department,
b.Name as Employee,
b.Salary as Salary
from (Department a straight_join Employee b on a.Id = b.DepartmentId) straight_join
(select c.DepartmentId,max(c.Salary) as Salary from (select * from Employee order by Salary desc) c group by c.DepartmentId) d
on a.Id = d.DepartmentId and b.Salary = d.Salary;
1、in 和 not in 也要慎用，否则会导致全表扫描
2、很多时候用 exists 代替 in 是一个好的选择
不过通过后面的优化，可以看出 in 确实挺慢的
在 MySQL 的 SELECT 查询当中，其核心算法就是 JOIN 查询算法。其他的查询语句都相应向 JOIN 靠拢：单表查询被当作 JOIN 的特例；子查询被尽量转换为 JOIN 查询
对于多表查询，如果可以确定表按照某一固定次序处理可以获得较好的效率，则建议加上 STRAIGHT_JOIN 子句，以减少优化器对表进行重排序优化的过程。
该子句一方面可以用于优化器无法给出最优排列的 SQL 语句；另一方面同样适用于优化器可以给出最优排列的 SQL 语句，因为 MySQL 算出最优排列也需要耗费较长的流程。
对于后一状况，可以根据 EXPLAIN 的提示选定表的顺序，并加上 STRAIGHT_JOIN 子句固定该顺序。该状况下的使用前提是几个表之间的数据量比例会一直保持在某一顺序，否则在各表数据此消彼长之后会适得其反。
对于经常调用的 SQL 语句，这一方法效果较好；同时操作的表越多，效果越好。
至此，优化还没完全结束，leetcode上该题最快是813ms，但是没有分享代码，最后贴两个别人家的代码：
Join twice，890ms accepted
SELECT Name, Employee, Salary FROM Department JOIN (SELECT Employee.Name AS Employee, Employee.Salary, Employee.DepartmentId FROM Employee JOIN (SELECT `DepartmentId`, MAX(`Salary`) AS Salary FROM `Employee` GROUP BY `DepartmentId` ) t1 ON t1.DepartmentId = Employee.DepartmentId AND t1.Salary = Employee.Salary ) t2 ON Department.Id = t2.DepartmentId
Easy Solution. No joins. GROUP BY is enough. 916ms
select d.Name, e.Name, e.Salary from Department d, Employee e, (select MAX(Salary) as Salary, DepartmentId as DepartmentId from Employee GROUP BY DepartmentId) h where e.Salary = h.Salary and e.DepartmentId = h.DepartmentId and e.DepartmentId = d.Id;
leetcode-184-Department Highest Salary 优化记录
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原文地址：http://www.cnblogs.com/zhangyunhao/p/4896055.html
