03dml查询

– 如果存在就删除名为hrs的数据库

drop database if exists `hrs`;

– 创建名为hrs的数据库并指定默认的字符集

create database `hrs` default charset utf8mb4;

– 切换到hrs数据库

use `hrs`;

– 创建部门表

create table `tb_dept`
(
`dno` int not null comment '编号',
`dname` varchar(10) not null comment '名称',
`dloc` varchar(20) not null comment '所在地',
primary key (`dno`)
);

– 插入4个部门

insert into `tb_dept` values (10, '会计部', '北京'),(20, '研发部', '成都'),(30, '销售部', '重庆'),(40, '运维部', '深圳');

– 创建员工表

create table `tb_emp`
(
`eno` int not null comment '员工编号',
`ename` varchar(20) not null comment '员工姓名',
`job` varchar(20) not null comment '员工职位',
`mgr` int comment '主管编号',
`sal` int not null comment '员工月薪',
`comm` int comment '每月补贴',
`dno` int not null comment '所在部门编号',
primary key (`eno`),
constraint `fk_emp_mgr` foreign key (`mgr`) references tb_emp (`eno`),
constraint `fk_emp_dno` foreign key (`dno`) references tb_dept (`dno`)
);

– 插入14个员工

insert into `tb_emp` values (7800, '张三丰', '总裁', null, 9000, 1200, 20),(2056, '乔峰', '分析师', 7800, 5000, 1500, 20),(3088, '李莫愁', '设计师', 2056, 3500, 800, 20),(3211, '张无忌', '程序员', 2056, 3200, null, 20),(3233, '丘处机', '程序员', 2056, 3400, null, 20),(3251, '张翠山', '程序员', 2056, 4000, null, 20),(5566, '宋远桥', '会计师', 7800, 4000, 1000, 10),(5234, '郭靖', '出纳', 5566, 2000, null, 10),(3344, '黄蓉', '销售主管', 7800, 3000, 800, 30),(1359, '胡一刀', '销售员', 3344, 1800, 200, 30),(4466, '苗人凤', '销售员', 3344, 2500, null, 30),(3244, '欧阳锋', '程序员', 3088, 3200, null, 20),(3577, '杨过', '会计', 5566, 2200, null, 10),(3588, '朱九真', '会计', 5566, 2500, null, 10);

– 查询员工和他的主管的姓名

select t1.ename as 姓名, t2.ename as 主管姓名 
from tb_emp as t1 left join tb_emp as t2 on t1.mgr = t2.eno;select t1.ename,t2.enamefrom tb_emp as t1
left join tb_emp as t2on t1.mgr = t2.eno;

– 查询月薪最高的员工姓名和月薪

-- 方法一：
select ename as 姓名,salfrom tb_emporder by sal desclimit 1;
-- 方法二：
select ename,sal
from tb_emp
where sal = (select max(sal) from tb_emp);
-- 方法三：
-- all / any 
SELECT ename, sal
FROM tb_emp
WHERE sal >= ALL (SELECT salFROM tb_emp);select ename,sal from tb_emp as t1
where (select count(*) from tb_emp as t2where t1.sal t1.sal
) = 0;
-- eg:月薪 top 3
select ename,sal
from tb_emp as t1
where (select count(*)from tb_emp as t2where t2.sal > t1.sal
) < 3;
-- 月薪 最低的3人
select ename,sal
from tb_emp as t1
where (select count(*)from tb_emp as t2where t2.sal < t1.sal
) < 3;-- 方法五：存在性判断 exists / not existsselect ename,salfrom tb_emp as t1where not exists(select sal /*此处查什么都可以，例如1，x , ename 都可*/from tb_emp as t2where t2.sal > t1.sal);
-- eg:存在性判断例题
select now() from dual/*此处的from dual 可以省略*/
where exists (select 'x',100 from tb_empwhere dno=20);select ename,sal
from tb_emp as t1
where exists(select 'x' from tb_emp as t2where t1.sal < t2.sal);

– 查询月薪Top3的员工姓名和月薪

select ename,sal from tb_emp
order by sal desc
limit 3;
select ename,sal from tb_emp as t1
where (select count(*)from tb_emp as t2where t2.sal>t1.sal)<3;

– 查询员工的姓名和年薪(年薪=(sal+comm)*13)

SELECT ename AS 姓名, (sal + (COALESCE(comm, 0))) * 13 AS 年薪
FROM tb_emp
ORDER BY 年薪 DESC;

– 查询部门的编号和人数

SELECT dno, COUNT(*)
FROM tb_emp
GROUP BY dno;

– 查询部门人数超过5个人的部门的编号和人数

select dno,count(*) as 人数 from tb_emp
group by dno
having 人数 > 5;

– 查询所有部门的名称和人数

select dname,coalesce(人数,0)
from tb_dept left join(select dno,count(*) as 人数 from tb_empgroup by dno) as tmpon tb_dept.dno = tmp.dno;
-- 方法一：
select dname as 部门名称,count(eno) as 部门人数 
from tb_dept as t1left join tb_emp as t2on t1.dno = t2.dno
group by dname;
-- 方法二：
select dname as 部门名称,coalesce(total,0) as 部门人数
from tb_dept as t1left join (select dno,count(*) as totalfrom tb_empgroup by dno) as t2on t1.dno = t2.dno;

– 查询月薪超过平均月薪的员工的姓名和月薪

select ename,sal from tb_emp
where tb_emp.sal > (select round(avg(sal),2) from tb_emp);

– 查询月薪超过其所在部门平均月薪的员工的姓名、部门编号和月薪

SELECT ename,sal,dno 
FROM tb_emp as t1 natural join (select dno,avg(sal) as avg_sal from tb_empgroup by dno) as t2
where sal > avg_sal;

– 查询部门中月薪最高的人姓名、月薪和所在部门名称

SELECT ename, sal,dname 
FROM tb_deptNATURAL JOIN (SELECT ename, sal,t1.dno FROM tb_empINNER JOIN ( SELECT dno, max( sal ) AS max_sal FROM tb_emp GROUP BY dno ) AS t1 ON tb_emp.dno = t1.dno AND tb_emp.sal = t1.max_sal ) AS t2;select ename, sal, dnamefrom tb_emp as t1natural join tb_dept as t2natural join (select dno,max(sal) as max_salfrom tb_empgroup by dno) as t3where sal = max_sal;delete ename,sal,dname
from tb_dept as t1natural join tb_emp as t2
where (dno,sal) in (select dno,max(sal)from tb_empgroup by dno);

– 查询主管的姓名和职位

SELECT ename, job
FROM tb_emp
WHERE eno IN (SELECT DISTINCT mgrFROM tb_empWHERE mgr IS NOT NULL);/*性能差*/select ename, job from tb_emp as t1
where exists (select 'X' from tb_emp as t2where t1.eno = t2.mgr);

– 查询普通员工（不是主管）的姓名和职位

SELECT ename, job
FROM tb_emp
WHERE eno not IN (SELECT DISTINCT mgrFROM tb_empWHERE mgr IS NOT NULL);select ename, job from tb_emp as t1
where not exists (select 'X' from tb_emp as t2where t1.eno = t2.mgr);

– 查询月薪排名4~6名的员工排名、姓名和月薪

select @a := 0;/*定义变量*/
-- select @a := @a + 2;select * from (select ename, sal, @a := @a + 1 as 排名from tb_emp,(select @a := 0) as tmporder by sal desc) as tmp
where 排名 between 4 and 6;

– 窗口函数，分析函数（性能很差，适合数据分析，不适开发），sql 8

select * from(
select ename,sal,row_number() over (order by sal desc) as 排名1,rank() over (order by sal desc) as 排名2,dense_rank() over (order by sal desc) as 排名3from tb_emp)as tmpwhere 排名3 between 4 and 6;

– 查询每个部门月薪排前2名的员工姓名、月薪和部门编号

select ename,sal,t1.dno from tb_emp as t1
where (select count(*)from tb_emp as t2where t2.dno = t1.dno and t1.sal > t2.sal) < 2
order by dno asc,sal desc;select ename,sal,dno from(
select ename,sal,dno,rank() over (partition by dno order by sal desc) as rnfrom tb_emp) as tmp
where rn <= 2;

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