数据库实例查询

drop database if exists hrs;
create database hrs default charset utf8;

use hrs;

drop table if exists tb_emp;
drop table if exists tb_dept;

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)
);

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');

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 comment '所在部门编号',
primary key (eno)
);

alter table tb_emp add constraint fk_emp_dno foreign key (dno) references tb_dept (dno);

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);

-- 查询月薪最高的员工姓名和工资（子查询/嵌套查询+聚合函数）
select ename, sal from tb_emp where sal=(select max(sal) from tb_emp);

-- 查询员工的姓名和年薪((月薪+补贴)13)
select ename as 姓名, (sal+ifnull(comm, 0))13 as 年薪 from tb_emp order by 年薪 desc;

-- 查询有员工的部门的编号和人数（分组查询+聚合函数）
select dno, count(dno) from tb_emp group by dno with rollup;

-- 查询所有部门的名称和人数（子查询+连接查询（左外））
select dname as 部门名称, ifnull(total, 0) as 人数 from tb_dept t1 left outer join (select dno, count(dno) as total from tb_emp group by dno) t2 on t1.dno=t2.dno;

-- 查询月薪最高的员工(Boss除外)的姓名和工资（空值判断）
select ename, sal from tb_emp where sal=(select max(sal) from tb_emp where mgr is not null);

-- 查询月薪超过平均薪水的员工的姓名和工资（子查询）
select ename, sal from tb_emp where sal>(select avg(sal) as avgsal from tb_emp);

-- 查询月薪超过其所在部门平均薪水的员工的姓名、部门编号和工资（分组查询+聚合函数+子查询+连接查询）
select ename, t1.dno, sal from tb_emp t1 inner join (select dno, avg(sal) as avgsal from tb_emp group by dno) t2 on t1.dno=t2.dno where sal>avgsal;

-- 查询部门中薪水最高的人姓名、工资和所在部门名称
select ename, sal, dname from (select ename, sal, t1.dno from tb_emp t1 inner join (select dno, max(sal) as maxsal from tb_emp group by dno) t2 on t1.dno=t2.dno where sal=maxsal) t3 inner join tb_dept t4 on t3.dno=t4.dno;

-- 查询主管的姓名和职位（去重和集合运算）
select ename, job from tb_emp where eno in (select distinct mgr from tb_emp where mgr is not null);

-- 查询月薪排名4~6名的员工姓名和工资
select ename, sal from tb_emp order by sal desc limit 3, 3;
select ename, sal from tb_emp order by sal desc limit 3 offset 3;

顺序
---from 子句：
-- where子句
--group by 子句
--having 子句
---order by 子句
--- limit 子句

创建索引（加速查询，索引通常创建在经常作为帅选条件的列上）
create index idx_emp_ename on tb_emp (ename)

-- 查询所有学生信息
select * from tb_student;

-- 查询所有课程名称及学分(投影和别名)
select couname, coucredit from tb_course;
select couname as 课程名称, coucredit as 学分 from tb_course;

-- 查询所有学生的姓名和性别(条件运算)
select stuname as 姓名, case stusex when 1 then '男' else '女' end as 性别 from tb_student;
select stuname as 姓名, if(stusex, '男', '女') as 性别 from tb_student;

-- 查询所有女学生的姓名和出生日期(筛选)
select stuname, stubirth from tb_student where stusex=0;

-- 查询所有80后学生的姓名、性别和出生日期(筛选)
select stuname, stusex, stubirth from tb_student where stubirth>='1980-1-1' and stubirth<='1989-12-31';
select stuname, stusex, stubirth from tb_student where stubirth between '1980-1-1' and '1989-12-31';

-- 查询姓"杨"的学生姓名和性别(模糊)
select stuname, stusex from tb_student where stuname like '杨%';

-- 查询姓"杨"名字两个字的学生姓名和性别(模糊)
select stuname, stusex from tb_student where stuname like '杨_';

-- 查询姓"杨"名字三个字的学生姓名和性别(模糊)
select stuname, stusex from tb_student where stuname like '杨__';

-- 查询名字中有"不"字或"嫣"字的学生的姓名(模糊)
select stuname, stusex from tb_student where stuname like '%不%' or stuname like '%嫣%';

-- 查询没有录入家庭住址的学生姓名(空值)
select stuname from tb_student where stuaddr is null;

-- 查询录入了家庭住址的学生姓名(空值)
select stuname from tb_student where stuaddr is not null;

-- 查询学生选课的所有日期(去重)
select distinct seldate from tb_record;

-- 查询学生的家庭住址(去重)
select distinct stuaddr from tb_student where stuaddr is not null;

-- 查询男学生的姓名和生日按年龄从大到小排列(排序)
select stuname as 姓名, datediff(curdate(), stubirth) div 365 as 年龄 from tb_student where stusex=1 order by 年龄 desc;

-- 查询年龄最大的学生的出生日期(聚合函数)
select min(stubirth) from tb_student;

-- 查询年龄最小的学生的出生日期(聚合函数)
select max(stubirth) from tb_student;

-- 查询男女学生的人数(分组和聚合函数)
select stusex, count(*) from tb_student group by stusex;

-- 查询课程编号为1111的课程的平均成绩(筛选和聚合函数)
select avg(score) from tb_record where cid=1111;

-- 查询学号为1001的学生所有课程的平均分(筛选和聚合函数)
select avg(score) from tb_record where sid=1001;

-- 查询每个学生的学号和平均成绩(分组和聚合函数)
select sid as 学号, avg(score) as 平均分 from tb_record group by sid;

-- 查询平均成绩大于等于90分的学生的学号和平均成绩
-- 分组以前的筛选使用where子句 / 分组以后的筛选使用having子句
select sid as 学号, avg(score) as 平均分 from tb_record group by sid having 平均分>=90;

-- 查询年龄最大的学生的姓名(子查询/嵌套的查询)
select stuname from tb_student where stubirth=( select min(stubirth) from tb_student );

-- 查询年龄最大的学生姓名和年龄(子查询+运算)
select stuname as 姓名, datediff(curdate(), stubirth) div 365 as 年龄 from tb_student where stubirth=( select min(stubirth) from tb_student );

-- 查询选了两门以上的课程的学生姓名(子查询/分组条件/集合运算)
select stuname from tb_student where stuid in ( select stuid from tb_record group by stuid having count(stuid)>2 );