假定業(yè)務(wù)：

查看在職員工的薪資的第二名的員工信息

創(chuàng)建數(shù)據(jù)庫

drop database if exists emps;create database emps;use emps;create table employees( empId int primary key,-- 員工編號(hào) gender char(1) NOT NULL, -- 員工性別hire_date date NOT NULL -- 員工入職時(shí)間 );create table salaries( empId int primary key, salary double -- 員工薪資 ); 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);

題解思路

1、(基礎(chǔ)解法)

先查出salaries表中最高薪資,再以此為條件查出第二高的工資

查詢語句如下：

selectE.empId,E.gender,E.hire_date,S.salaryfromemployees E join salaries S on E.empId = S.empIdwhereS.salary=( select max(salary)from salaries where salary<(select max(salary) from salaries) );-- ---------------查詢結(jié)果------------ --+-------+--------+------------+--------+| empId | gender | hire_date | salary |+-------+--------+------------+--------+| 10004 | M | 1986-12-01 | 74057 |+-------+--------+------------+--------+

2、(自聯(lián)結(jié)查詢)

先對(duì)salaries進(jìn)行自聯(lián)結(jié)查詢，當(dāng)s1<=s2鏈接并以s1.salary分組，此時(shí)count的值，即薪資比他高的人數(shù)，用having篩選count=2 的人，就可以得到第二高的薪資了；

查詢語句如下：

selectE.empId,E.gender,E.hire_date,S.salaryfromemployees E join salaries S on E.empId = S.empIdwhere S.salary=( select s1.salary from salaries s1 join salaries s2 on s1.salary <= s2.salary group by s1.salaryhaving count(distinct s2.salary) = 2 );-- ---------------查詢結(jié)果------------ --+-------+--------+------------+--------+| empId | gender | hire_date | salary |+-------+--------+------------+--------+| 10004 | M | 1986-12-01 | 74057 |+-------+--------+------------+--------+

3、(自聯(lián)結(jié)查詢優(yōu)化版)

原理和2相同，但是代碼精簡了很多，上面兩種是為了引出最后這種方法，在很多時(shí)候group by和order by都有其局限性，對(duì)于俺們初學(xué)者掌握這種實(shí)用性較廣的思路，還是很有意義的。

selectE.empId,E.gender,E.hire_date,S.salaryfromemployees E join salaries S on S.empId =E.empIdwhere (select count(1) from salaries where salary>=S.salary)=2；-- ---------------查詢結(jié)果------------ --+-------+--------+------------+--------+| empId | gender | hire_date | salary |+-------+--------+------------+--------+| 10004 | M | 1986-12-01 | 74057 |+-------+--------+------------+--------+

初淺總結(jié)，如有錯(cuò)誤，還望指正。

總結(jié)

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