多表查询

　　准备工作：准备两张表，部门表（department）、员工表（employee）

create table department(
id int,
name varchar(20) 
);

create table employee(
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 department values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');

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

# 查看表结构和数据
mysql> desc department;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.19 sec)

mysql> desc employee;
+--------+-----------------------+------+-----+---------+----------------+
| 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    |                |
+--------+-----------------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)

mysql> select * from department;
+------+--------------+
| id   | name         |
+------+--------------+
|  200 | 技术         |
|  201 | 人力资源     |
|  202 | 销售         |
|  203 | 运营         |
+------+--------------+
4 rows in set (0.02 sec)

mysql> select * from employee;
+----+----------+--------+------+--------+
| 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 | nvshen   | male   |   18 |    200 |
|  6 | xiaomage | female |   18 |    204 |
+----+----------+--------+------+--------+
6 rows in set (0.00 sec)

多表连接查询：

　　两张表的准备工作已完成，比如现在我要查询的员工信息以及该员工所在的部门。从该题中，我们看出既要查员工又要查该员工的部门，肯定要将两张表进行连接查询，多表连接查询。

　　外连接语法：

　　

select 字段列表
    from 表1 inner|left|right join 表2 on 表1.字段 = 表2.字段；

　　（1）先看第一种情况交叉连接：不适用任何匹配条件。生成笛卡尔积（关于笛卡尔积的含义，大家百度自行补脑）。

mysql> select * from employe,department;
+----+----------+--------+------+--------+------+--------------+
| id | name     | sex    | age  | dep_id | id   | name         |
+----+----------+--------+------+--------+------+--------------+
|  1 | egon     | male   |   18 |    200 |  200 | 技术         |
|  1 | egon     | male   |   18 |    200 |  201 | 人力资源     |
|  1 | egon     | male   |   18 |    200 |  202 | 销售         |
|  1 | egon     | male   |   18 |    200 |  203 | 运营         |
|  2 | alex     | female |   48 |    201 |  200 | 技术         |
|  2 | alex     | female |   48 |    201 |  201 | 人力资源     |
|  2 | alex     | female |   48 |    201 |  202 | 销售         |
|  2 | alex     | female |   48 |    201 |  203 | 运营         |
|  3 | wupeiqi  | male   |   38 |    201 |  200 | 技术         |
|  3 | wupeiqi  | male   |   38 |    201 |  201 | 人力资源     |
|  3 | wupeiqi  | male   |   38 |    201 |  202 | 销售         |
|  3 | wupeiqi  | male   |   38 |    201 |  203 | 运营         |
|  4 | yuanhao  | female |   28 |    202 |  200 | 技术         |
|  4 | yuanhao  | female |   28 |    202 |  201 | 人力资源     |
|  4 | yuanhao  | female |   28 |    202 |  202 | 销售         |
|  4 | yuanhao  | female |   28 |    202 |  203 | 运营         |
|  5 | nvshen   | male   |   18 |    200 |  200 | 技术         |
|  5 | nvshen   | male   |   18 |    200 |  201 | 人力资源     |
|  5 | nvshen   | male   |   18 |    200 |  202 | 销售         |
|  5 | nvshen   | male   |   18 |    200 |  203 | 运营         |
|  6 | xiaomage | female |   18 |    204 |  200 | 技术         |
|  6 | xiaomage | female |   18 |    204 |  201 | 人力资源     |
|  6 | xiaomage | female |   18 |    204 |  202 | 销售         |
|  6 | xiaomage | female |   18 |    204 |  203 | 运营         |
+----+----------+--------+------+--------+------+--------------+
24 rows in set (0.00 sec)

（2）内连接：只连接匹配的行

　　　　找到两张表共有的部分，相当于利用条件从笛卡儿积结果中筛选出了匹配的结果。

　　　　department没有204这个部门，因而employe表中关于204这条员工信息没有匹配出来。

mysql> select employe.id,employe.name,employe.age,employe.sex,department.name from employe inner join department on employe.dep_id=department.id;
+----+---------+------+--------+--------------+
| id | name    | age  | sex    | name         |
+----+---------+------+--------+--------------+
|  1 | egon    |   18 | male   | 技术         |
|  2 | alex    |   48 | female | 人力资源     |
|  3 | wupeiqi |   38 | male   | 人力资源     |
|  4 | yuanhao |   28 | female | 销售         |
|  5 | nvshen  |   18 | male   | 技术         |
+----+---------+------+--------+--------------+
5 rows in set (0.00 sec)

##上述sql等同于
mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;

　　

（3）外链接之左连接：优先显示左表全部记录

　　　　以左表为准，即找出所有员工信息，当然包括没有部门的员工。本质就是，在内连接的基础上增加左边有，右边灭有的结果。

mysql> select employe.id,employe.name,department.name as depart_name from employe left join department on employe.dep_id = department.id;
+----+----------+--------------+
| id | name     | depart_name  |
+----+----------+--------------+
|  1 | egon     | 技术         |
|  5 | nvshen   | 技术         |
|  2 | alex     | 人力资源     |
|  3 | wupeiqi  | 人力资源     |
|  4 | yuanhao  | 销售         |
|  6 | xiaomage | NULL         |
+----+----------+--------------+
6 rows in set (0.00 sec)

（4） 外链接之右连接：优先显示右表全部记录

　　　　以右表为准，即找出所有部门信息，包括没有员工的部门，本质就是，在内连接的基础上增加右边有，左边没有的结果。

mysql> select employe.id,employe.name,department.name as depart_name from employe right join department on employe.dep_id = department.id;
+------+---------+--------------+
| id   | name    | depart_name  |
+------+---------+--------------+
|    1 | egon    | 技术         |
|    2 | alex    | 人力资源     |
|    3 | wupeiqi | 人力资源     |
|    4 | yuanhao | 销售         |
|    5 | nvshen  | 技术         |
| NULL | NULL    | 运营         |
+------+---------+--------------+
6 rows in set (0.00 sec)

（5） 全外连接：显示左右两个表全部记录(了解)

　　　　外连接：在内连接的基础上增加左边有，右边没有的或者是右边有，左边没有的结果。

　　　　注意：mysql不支持全外连接  full join

　　　　强调：mysql可以使用以下方式间接实现全外连接。

　　语法：select * from employee left join department on employee.dep_id = department.id union all select * from employee right join department on employee.dep_id = department.id;

　　　　   select * from employee left join department on employee.dep_id = department.id union select * from employee right join department on employee.dep_id = department.id;

　　　　union 与 union all 的区别：union会去掉相同的记录。

mysql> select * from employe left join department on employe.dep_id = department.id union all select * from employe right join department on employe.dep_id = department.id;
+------+----------+--------+------+--------+------+--------------+
| id   | name     | sex    | age  | dep_id | id   | name         |
+------+----------+--------+------+--------+------+--------------+
|    1 | egon     | male   |   18 |    200 |  200 | 技术         |
|    5 | nvshen   | male   |   18 |    200 |  200 | 技术         |
|    2 | alex     | female |   48 |    201 |  201 | 人力资源     |
|    3 | wupeiqi  | male   |   38 |    201 |  201 | 人力资源     |
|    4 | yuanhao  | female |   28 |    202 |  202 | 销售         |
|    6 | xiaomage | female |   18 |    204 | NULL | NULL         |
|    1 | egon     | male   |   18 |    200 |  200 | 技术         |
|    2 | alex     | female |   48 |    201 |  201 | 人力资源     |
|    3 | wupeiqi  | male   |   38 |    201 |  201 | 人力资源     |
|    4 | yuanhao  | female |   28 |    202 |  202 | 销售         |
|    5 | nvshen   | male   |   18 |    200 |  200 | 技术         |
| NULL | NULL     | NULL   | NULL |   NULL |  203 | 运营         |
+------+----------+--------+------+--------+------+--------------+
12 rows in set (0.01 sec)

mysql> select * from employe left join department on employe.dep_id = department.id union select * from employe right join department on employe.dep_id = department.id;
+------+----------+--------+------+--------+------+--------------+
| id   | name     | sex    | age  | dep_id | id   | name         |
+------+----------+--------+------+--------+------+--------------+
|    1 | egon     | male   |   18 |    200 |  200 | 技术         |
|    5 | nvshen   | male   |   18 |    200 |  200 | 技术         |
|    2 | alex     | female |   48 |    201 |  201 | 人力资源     |
|    3 | wupeiqi  | male   |   38 |    201 |  201 | 人力资源     |
|    4 | yuanhao  | female |   28 |    202 |  202 | 销售         |
|    6 | xiaomage | female |   18 |    204 | NULL | NULL         |
| NULL | NULL     | NULL   | NULL |   NULL |  203 | 运营         |
+------+----------+--------+------+--------+------+--------------+
7 rows in set (0.00 sec)

符合条件查询：

　　找出年龄大于25岁的员工所在的部门：

mysql> select employe.name,employe.age,department.name from employe join department on employe.dep_id = department.id where age>25;
+---------+------+--------------+
| name    | age  | name         |
+---------+------+--------------+
| alex    |   48 | 人力资源     |
| wupeiqi |   38 | 人力资源     |
| yuanhao |   28 | 销售         |
+---------+------+--------------+
3 rows in set (0.00 sec)

　　以内链接的方式查询employe 和 department 表，并且以age字段的升序方式显示：

mysql> select * from employe join department on employe.dep_id = department.id order by age desc;
+----+---------+--------+------+--------+------+--------------+
| id | name    | sex    | age  | dep_id | id   | name         |
+----+---------+--------+------+--------+------+--------------+
|  2 | alex    | female |   48 |    201 |  201 | 人力资源     |
|  3 | wupeiqi | male   |   38 |    201 |  201 | 人力资源     |
|  4 | yuanhao | female |   28 |    202 |  202 | 销售         |
|  1 | egon    | male   |   18 |    200 |  200 | 技术         |
|  5 | nvshen  | male   |   18 |    200 |  200 | 技术         |
+----+---------+--------+------+--------+------+--------------+
5 rows in set (0.00 sec)

mysql>

子查询：

　　1，子查询是将一个查询语句嵌套在另一个查询语句中。

　　2，内层查询语句结果，可以为外层查询语句提供查询条件。

　　3，子查询中可以包含： in , not , in , any , all ,exists , not exists 等关键字。

　　4，还可以包含比较运算符： = ， != ，>，< 等

带 in 关键字的查询：

　　查询平均年龄在25岁以上的部门名。

mysql> select department.name from department where id in (select dep_id from employe where age > 25);
+--------------+
| name         |
+--------------+
| 人力资源     |
| 销售         |
+--------------+
2 rows in set (0.00 sec)

　　查看技术部员工的姓名：

mysql> select employe.name from employe where dep_id in (select id from department where department.name = '技术');
+--------+
| name   |
+--------+
| egon   |
| nvshen |
+--------+
2 rows in set (0.00 sec)

　　查看不足1人的部门：

（1）
mysql> select department.name from department where id not in (select dep_id from employe inner join department where department.id = employe.dep_id);
+--------+
| name   |
+--------+
| 运营   |
+--------+
1 row in set (0.00 sec)

（2）
mysql> select name from department where id not in (select dep_id from employe group by dep_id);
+--------+
| name   |
+--------+
| 运营   |
+--------+
1 row in set (0.00 sec)

带比较运算符的子查询：

　　查询大于所有人平均年龄的员工名与年龄：

mysql> select employe.name,employe.age from employe where age > (select avg(age) from employe);
+---------+------+
| name    | age  |
+---------+------+
| alex    |   48 |
| wupeiqi |   38 |
+---------+------+
2 rows in set (0.00 sec)

　　查询大于部门内平均年龄的员工名：

mysql> select employe.name,employe.age from employe inner join (select dep_id,avg(age) as b from employe group by dep_id) as A on employe.dep_id = A.dep_id where employe.age>A.b;
+------+------+
| name | age  |
+------+------+
| alex |   48 |
+------+------+
1 row in set (0.00 sec)

##注意：where 后不能直接用聚合函数，若想用就在之前为聚合函数取一个别名，然后再后面用这个别名调用。

带exists关键字的子查询：

　　exists 关键字表示存在，在使用exists关键字时，内层查询语句不返回查询记录，而是返回一个真假值。True或False。

　　当返回True时，外层查询语句将进行查询：当返回值为False时，外层查询语句不进行查询。

mysql> select * from employee  where exists (select id from department where id=200);
+----+----------+--------+------+--------+
| 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 | nvshen   | male   |   18 |    200 |
|  6 | xiaomage | female |   18 |    204 |
+----+----------+--------+------+--------+
#department表中存在dept_id=205，False
mysql> select * from employee  where exists (select id from department where id=204);
Empty set (0.00 sec)

查询每个部门中最新入职的员工：

create table employee(
id int not null unique auto_increment,
name varchar(20) not null,
sex enum('male','female') not null default 'male', #大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int, #一个部门一个屋子
depart_id int
);


#查看表结构
mysql> desc employee;
+--------------+-----------------------+------+-----+---------+----------------+
| Field        | Type                  | Null | Key | Default | Extra          |
+--------------+-----------------------+------+-----+---------+----------------+
| id           | int(11)               | NO   | PRI | NULL    | auto_increment |
| name         | varchar(20)           | NO   |     | NULL    |                |
| sex          | enum('male','female') | NO   |     | male    |                |
| age          | int(3) unsigned       | NO   |     | 28      |                |
| hire_date    | date                  | NO   |     | NULL    |                |
| post         | varchar(50)           | YES  |     | NULL    |                |
| post_comment | varchar(100)          | YES  |     | NULL    |                |
| salary       | double(15,2)          | YES  |     | NULL    |                |
| office       | int(11)               | YES  |     | NULL    |                |
| depart_id    | int(11)               | YES  |     | NULL    |                |
+--------------+-----------------------+------+-----+---------+----------------+

#插入记录
#三个部门：教学，销售，运营
insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values
('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部
('alex','male',78,'20150302','teacher',1000000.31,401,1),
('wupeiqi','male',81,'20130305','teacher',8300,401,1),
('yuanhao','male',73,'20140701','teacher',3500,401,1),
('liwenzhou','male',28,'20121101','teacher',2100,401,1),
('jingliyang','female',18,'20110211','teacher',9000,401,1),
('jinxin','male',18,'19000301','teacher',30000,401,1),
('成龙','male',48,'20101111','teacher',10000,401,1),

('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),

('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬银','female',18,'20130311','operation',19000,403,3),
('程咬铜','male',18,'20150411','operation',18000,403,3),
('程咬铁','female',18,'20140512','operation',17000,403,3)
;

（1）
mysql> select employee.name,employee.hire_date from employee where hire_date in (select max(hire_date) from employee group by post);
+--------+------------+
| name   | hire_date  |
+--------+------------+
| egon   | 2017-03-01 |
| alex   | 2015-03-02 |
| 格格   | 2017-01-27 |
| 张野   | 2016-03-11 |
+--------+------------+
4 rows in set (0.00 sec)

mysql>


（2）

select * from employee as t1
inner join
(select post,max(hire_date) as new_date from employee group by post) as t2
on t1.post=t2.post
where t1.hire_date=t2.new_date;