MySQL 数据库之多表查询与子查询

一、多表连接查询

#建表
create table dep(
id int,
name varchar(20) 
);

create table emp(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);

#插入数据
insert into dep values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');

insert into emp(name,sex,age,dep_id) values
('egon','male',18,200),
('alex','female',48,201),
('wupeiqi','male',38,201),
('yuanhao','female',28,202),
('liwenzhou','male',18,200),
('jingliyang','female',18,204)
;


#查看表结构和数据
mysql> desc dep;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+

mysql> desc emp;
+--------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+-----------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
| sex | enum('male','female') | NO | | male | |
| age | int(11) | YES | | NULL | |
| dep_id | int(11) | YES | | NULL | |
+--------+-----------------------+------+-----+---------+----------------+

mysql> select * from dep;
+------+--------------+
| id | name |
+------+--------------+
| 200 | 技术 |
| 201 | 人力资源 |
| 202 | 销售 |
| 203 | 运营 |
+------+--------------+

mysql> select * from emp;
+----+------------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+------------+--------+------+--------+
| 1 | egon | male | 18 | 200 |
| 2 | alex | female | 48 | 201 |
| 3 | wupeiqi | male | 38 | 201 |
| 4 | yuanhao | female | 28 | 202 |
| 5 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
+----+------------+--------+------+--------+

表dep与emp

外连接语法
select * from 表1 inner join 表2 on 表1.字段 = 表2.字段;

1、内连接：不适用任何匹配条件。生成笛卡尔积

#应用：
    select * from emp,dep where emp.dep_id = dep.id and dep.name = "技术"; # 不要用where做连表的活

    select * from emp inner join dep on emp.dep_id = dep.id
        where dep.name = "技术"
    ;

2、左连接：在内连接的基础上，保留左边没有对应关系的记录
select * from emp left join dep on emp.dep_id = dep.id;


3、右连接：在内连接的基础上，保留右边没有对应关系的记录
select * from emp right join dep on emp.dep_id = dep.id;


4、全连接：在内连接的基础上，保留左、右边没有对应关系的记录
select * from emp left join dep on emp.dep_id = dep.id
union
select * from emp right join dep on emp.dep_id = dep.id;

#补充：多表连接可以不断地与虚拟表连接

查找各部门最高工资
select t1.* from emp as t1
inner join
(select post,max(salary) as ms from emp group by post) as t2
on t1.post = t2.post
where t1.salary = t2.ms
;

二、子查询

子查询：把一个查询语句用括号括起来，当做另外一条查询语句的条件去用，称为子查询

select emp.name from emp inner join dep on emp.dep_id = dep.id where dep.name="技术";

select name from emp where dep_id =
(select id from dep where name="技术");


查询平均年龄在25岁以上的部门名
select name from dep where id in
(select dep_id from emp group by dep_id having avg(age) > 25);

select dep.name from emp inner join dep on emp.dep_id = dep.id
    group by dep.name
    having avg(age) > 25;

查看不足2人的部门名(子查询得到的是有人的部门id)

select * from emp where exists (
    select id from dep where id > 3
);

查询每个部门最新入职的那位员工

select t1.id,t1.name,t1.post,t1.hire_date,t2.post,t2.max_date from emp as t1 inner join
(select post,max(hire_date) as max_date from emp group by post) as t2
on t1.post = t2.post
where t1.hire_date = t2.max_date
;