MySql多表查询

@[TOC]

1.准备表

#建表
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),
('liwenzhou','male',18,200),
('jingliyang','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 | |
+-------+-------------+------+-----+---------+-------+

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 | |
+--------+-----------------------+------+-----+---------+----------------+

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

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 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
+----+------------+--------+------+--------+

表department与employee

sql示例

2. 多表连接查询

#重点：外链接语法

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

• 交叉连接：不适用任何匹配条件。生成笛卡尔积

  mysql> select * from employee,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 | liwenzhou  | male   |   18 |    200 |  200 | 技术         |
  |  5 | liwenzhou  | male   |   18 |    200 |  201 | 人力资源     |
  |  5 | liwenzhou  | male   |   18 |    200 |  202 | 销售         |
  |  5 | liwenzhou  | male   |   18 |    200 |  203 | 运营         |
  |  6 | jingliyang | female |   18 |    204 |  200 | 技术         |
  |  6 | jingliyang | female |   18 |    204 |  201 | 人力资源     |
  |  6 | jingliyang | female |   18 |    204 |  202 | 销售         |
  |  6 | jingliyang | female |   18 |    204 |  203 | 运营         |
  +----+------------+--------+------+--------+------+--------------+
  

• 内连接：只连接匹配的行

  #找两张表共有的部分，相当于利用条件从笛卡尔积结果中筛选出了正确的结果
  #department没有204这个部门，因而employee表中关于204这条员工信息没有匹配出来
  mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.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 | liwenzhou |   18 | male   | 技术         |
  +----+-----------+------+--------+--------------+
  
  #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;
  

• 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没有的结果
  mysql> select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id;
  +----+------------+--------------+
  | id | name       | depart_name  |
  +----+------------+--------------+
  |  1 | egon       | 技术         |
  |  5 | liwenzhou  | 技术         |
  |  2 | alex       | 人力资源     |
  |  3 | wupeiqi    | 人力资源     |
  |  4 | yuanhao    | 销售         |
  |  6 | jingliyang | NULL         |
  +----+------------+--------------+
  

• 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 employee.id,employee.name,department.name as depart_name from employee right join department on employee.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 | liwenzhou | 技术         |
  | NULL | NULL      | 运营         |
  +------+-----------+--------------+
  

• 全外连接：显示左右两个表全部记录

  全外连接：在内连接的基础上增加左边有右边没有的和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可以使用此种方式间接实现全外连接
  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
  ;
  #查看结果
  +------+------------+--------+------+--------+------+--------------+
  | id   | name       | sex    | age  | dep_id | id   | name         |
  +------+------------+--------+------+--------+------+--------------+
  |    1 | egon       | male   |   18 |    200 |  200 | 技术         |
  |    5 | liwenzhou  | 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 | jingliyang | female |   18 |    204 | NULL | NULL        |
  | NULL | NULL       | NULL   | NULL |   NULL |  203 | 运营         |
  +------+------------+--------+------+--------+------+--------------+
  
  #注意 union与union all的区别：union会去掉相同的纪录
  

3. 符合条件连接查询

示例1：以内连接的方式查询employee和department表，并且employee表中的age字段值必须大于25,即找出年龄大于25岁的员工以及员工所在的部门

select employee.name,department.name from employee inner join department
    on employee.dep_id = department.id
    where age > 25;

示例2：以内连接的方式查询employee和department表，并且以age字段的升序方式显示

select employee.id,employee.name,employee.age,department.name from employee,department
    where employee.dep_id = department.id
    and age > 25
    order by age asc;

4. 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：内层查询语句的查询结果，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 和 NOT EXISTS等关键字

4：还可以包含比较运算符：= 、 !=、> 、<等

• 带IN关键字的子查询

  #查询平均年龄在25岁以上的部门名
  select id,name from department
      where id in 
          (select dep_id from employee group by dep_id having avg(age) > 25);
  
  #查看技术部员工姓名
  select name from employee
      where dep_id in 
          (select id from department where name='技术');
  
  #查看不足1人的部门名(子查询得到的是有人的部门id)
  select name from department where id not in (select distinct dep_id from employee);
  

• 带比较运算符的子查询

  #比较运算符：=、!=、>、>=、<、<=、<>
  #查询大于所有人平均年龄的员工名与年龄
  mysql> select name,age from emp where age > (select avg(age) from emp);
  +---------+------+
  | name    | age  |
  +---------+------+
  | alex    | 48   |
  | wupeiqi | 38   |
  +---------+------+
  rows in set (0.00 sec)
  
  
  #查询大于部门内平均年龄的员工名、年龄
  select t1.name,t1.age from emp t1
  inner join 
  (select dep_id,avg(age) avg_age from emp group by dep_id) t2
  on t1.dep_id = t2.dep_id
  where t1.age > t2.avg_age;
  

• 带EXISTS关键字的子查询
  EXISTS关字键字表示存在。在使用EXISTS关键字时，内层查询语句不返回查询的记录。
  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

  #department表中存在dept_id=203，Ture
  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 | liwenzhou  | male   |   18 |    200 |
  |  6 | jingliyang | 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)
  

5. 练习：查询每个部门最新入职的那位员工

• 创建表:

  company.employee
      员工id      id                  int             
      姓名        emp_name            varchar
      性别        sex                 enum
      年龄        age                 int
      入职日期     hire_date           date
      岗位        post                varchar
      职位描述     post_comment        varchar
      薪水        salary              double
      办公室       office              int
      部门编号     depart_id           int
  
  
  
  #创建表
  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)
  ;
  
  #ps：如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbk
  
  准备表和记录
  
  表与数据准备
  

• 答案1连表查询

  SELECT
      *
  FROM
      emp AS t1
  INNER JOIN (
      SELECT
          post,
          max(hire_date) max_date
      FROM
          emp
      GROUP BY
          post
  ) AS t2 ON t1.post = t2.post
  WHERE
      t1.hire_date = t2.max_date;
  
  答案一（连表查询）
  

• 答案2

  mysql> select (select t2.name from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post;
  +---------------------------------------------------------------------------------------+
  | (select t2.name from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) |
  +---------------------------------------------------------------------------------------+
  | 张野                                                                                  |
  | 格格                                                                                  |
  | alex                                                                                  |
  | egon                                                                                  |
  +---------------------------------------------------------------------------------------+
  rows in set (0.00 sec)
  
  mysql> select (select t2.id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post;
  +-------------------------------------------------------------------------------------+
  | (select t2.id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) |
  +-------------------------------------------------------------------------------------+
  |                                                                                  14 |
  |                                                                                  13 |
  |                                                                                   2 |
  |                                                                                   1 |
  +-------------------------------------------------------------------------------------+
  rows in set (0.00 sec)
  
  #正确答案
  mysql> select t3.name,t3.post,t3.hire_date from emp as t3 where id in (select (select id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post);
  +--------+-----------------------------------------+------------+
  | name   | post                                    | hire_date  |
  +--------+-----------------------------------------+------------+
  | egon   | 老男孩驻沙河办事处外交大使              | 2017-03-01 |
  | alex   | teacher                                 | 2015-03-02 |
  | 格格   | sale                                    | 2017-01-27 |
  | 张野   | operation                               | 2016-03-11 |
  +--------+-----------------------------------------+------------+
  rows in set (0.00 sec)
  
  答案二（子查询）