假定业务：
查看在职员工的薪资的第二名的员工信息
创建数据库
drop database if exists emps;
create database emps;
use emps;
create table employees(
empId int primary key,-- 员工编号
gender char(1) NOT NULL, -- 员工性别
hire_date date NOT NULL -- 员工入职时间
);
create table salaries(
empId int primary key,
salary double -- 员工薪资
);
INSERT INTO employees VALUES(10001,'M','1986-06-26');
INSERT INTO employees VALUES(10002,'F','1985-11-21');
INSERT INTO employees VALUES(10003,'M','1986-08-28');
INSERT INTO employees VALUES(10004,'M','1986-12-01');
INSERT INTO salaries VALUES(10001,88958);
INSERT INTO salaries VALUES(10002,72527);
INSERT INTO salaries VALUES(10003,43311);
INSERT INTO salaries VALUES(10004,74057);
题解思路
1、(基础解法)
先查出salaries表中最高薪资,再以此为条件查出第二高的工资
查询语句如下：
select
E.empId,E.gender,E.hire_date,S.salary
from
employees E join salaries S
on
E.empId = S.empId
where
S.salary=
(
select max(salary)from salaries
where
salary<
(select max(salary) from salaries)
);
-- ---------------查询结果------------ --
+-------+--------+------------+--------+
| empId | gender | hire_date | salary |
+-------+--------+------------+--------+
| 10004 | M | 1986-12-01 | 74057 |
+-------+--------+------------+--------+
2、(自联结查询)
先对salaries进行自联结查询，当s1<=s2链接并以s1.salary分组，此时count的值，即薪资比他高的人数，用having筛选count=2 的人，就可以得到第二高的薪资了；
查询语句如下：
select
E.empId,E.gender,E.hire_date,S.salary
from
employees E join salaries S
on
E.empId = S.empId
where S.salary=
(
select
s1.salary
from
salaries s1 join salaries s2
on
s1.salary <= s2.salary
group by
s1.salary
having
count(distinct s2.salary) = 2
);
-- ---------------查询结果------------ --
+-------+--------+------------+--------+
| empId | gender | hire_date | salary |
+-------+--------+------------+--------+
| 10004 | M | 1986-12-01 | 74057 |
+-------+--------+------------+--------+
3、(自联结查询优化版)
原理和2相同，但是代码精简了很多，上面两种是为了引出最后这种方法，在很多时候group by和order by都有其局限性，对于俺们初学者掌握这种实用性较广的思路，还是很有意义的。
select
E.empId,E.gender,E.hire_date,S.salary
from
employees E join salaries S
on
S.empId =E.empId
where
(select count(1) from salaries where salary>=S.salary)=2；
-- ---------------查询结果------------ --
+-------+--------+------------+--------+
| empId | gender | hire_date | salary |
+-------+--------+------------+--------+
| 10004 | M | 1986-12-01 | 74057 |
+-------+--------+------------+--------+
初浅总结，如有错误，还望指正。
总结
