day07-多表查询

本节重点：

• 多表连接查询
• 符合条件连接查询
• 子查询

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
('tom','male',18,200),
('mike','female',48,201),
('jack','male',38,201),
('lucy','female',28,202),
('lili','male',18,200),
('alice','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 | tom   | male   |   18 |    200 |
|  2 | mike  | female |   48 |    201 |
|  3 | jack  | male   |   38 |    201 |
|  4 | lucy  | female |   28 |    202 |
|  5 | lili  | male   |   18 |    200 |
|  6 | alice | female |   18 |    204 |
+----+-------+--------+------+--------+
6 rows in set (0.00 sec)

ps:观察两张表，发现department表中id=203部门在employee中没有对应的员工，发现employee中id=6的员工在department表中没有对应关系。

一、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。

重点：外链接语法

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;

1.1、交叉连接

不适用任何匹配条件。生成笛卡尔积（简单的说就是两个集合相乘的结果）。

mysql> select * from employee, department;
+----+-------+--------+------+--------+------+--------------+
| id | name  | sex    | age  | dep_id | id   | name         |
+----+-------+--------+------+--------+------+--------------+
|  1 | tom   | male   |   18 |    200 |  200 | 技术         |
|  1 | tom   | male   |   18 |    200 |  201 | 人力资源     |
|  1 | tom   | male   |   18 |    200 |  202 | 销售         |
|  1 | tom   | male   |   18 |    200 |  203 | 运营         |
|  2 | mike  | female |   48 |    201 |  200 | 技术         |
|  2 | mike  | female |   48 |    201 |  201 | 人力资源     |
|  2 | mike  | female |   48 |    201 |  202 | 销售         |
|  2 | mike  | female |   48 |    201 |  203 | 运营         |
|  3 | jack  | male   |   38 |    201 |  200 | 技术         |
|  3 | jack  | male   |   38 |    201 |  201 | 人力资源     |
|  3 | jack  | male   |   38 |    201 |  202 | 销售         |
|  3 | jack  | male   |   38 |    201 |  203 | 运营         |
|  4 | lucy  | female |   28 |    202 |  200 | 技术         |
|  4 | lucy  | female |   28 |    202 |  201 | 人力资源     |
|  4 | lucy  | female |   28 |    202 |  202 | 销售         |
|  4 | lucy  | female |   28 |    202 |  203 | 运营         |
|  5 | lili  | male   |   18 |    200 |  200 | 技术         |
|  5 | lili  | male   |   18 |    200 |  201 | 人力资源     |
|  5 | lili  | male   |   18 |    200 |  202 | 销售         |
|  5 | lili  | male   |   18 |    200 |  203 | 运营         |
|  6 | alice | female |   18 |    204 |  200 | 技术         |
|  6 | alice | female |   18 |    204 |  201 | 人力资源     |
|  6 | alice | female |   18 |    204 |  202 | 销售         |
|  6 | alice | female |   18 |    204 |  203 | 运营         |
+----+-------+--------+------+--------+------+--------------+
24 rows in set (0.06 sec)

1.2、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匹配的结果
#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;
或
mysql> select * from employee inner join department on employee.dep_id = department.id;
+----+------+--------+------+--------+------+--------------+
| id | name | sex    | age  | dep_id | id   | name         |
+----+------+--------+------+--------+------+--------------+
|  1 | tom  | male   |   18 |    200 |  200 | 技术         |
|  2 | mike | female |   48 |    201 |  201 | 人力资源     |
|  3 | jack | male   |   38 |    201 |  201 | 人力资源     |
|  4 | lucy | female |   28 |    202 |  202 | 销售         |
|  5 | lili | male   |   18 |    200 |  200 | 技术         |
+----+------+--------+------+--------+------+--------------+
5 rows in set (0.05 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;

1.3、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，右边没有的结果

mysql> select * from employee left join department on employee.dep_id = department.id;
+----+-------+--------+------+--------+------+--------------+
| id | name  | sex    | age  | dep_id | id   | name         |
+----+-------+--------+------+--------+------+--------------+
|  1 | tom   | male   |   18 |    200 |  200 | 技术         |
|  5 | lili  | male   |   18 |    200 |  200 | 技术         |
|  2 | mike  | female |   48 |    201 |  201 | 人力资源     |
|  3 | jack  | male   |   38 |    201 |  201 | 人力资源     |
|  4 | lucy  | female |   28 |    202 |  202 | 销售         |
|  6 | alice | female |   18 |    204 | NULL | NULL         |
+----+-------+--------+------+--------+------+--------------+
6 rows in set (0.00 sec)

1.4、外链接之右连接：优先显示右表全部记录

mysql> select * from employee right join department on employee.dep_id = department.id;
+------+------+--------+------+--------+------+--------------+
| id   | name | sex    | age  | dep_id | id   | name         |
+------+------+--------+------+--------+------+--------------+
|    1 | tom  | male   |   18 |    200 |  200 | 技术         |
|    2 | mike | female |   48 |    201 |  201 | 人力资源     |
|    3 | jack | male   |   38 |    201 |  201 | 人力资源     |
|    4 | lucy | female |   28 |    202 |  202 | 销售         |
|    5 | lili | male   |   18 |    200 |  200 | 技术         |
| NULL | NULL | NULL   | NULL |   NULL |  203 | 运营         |
+------+------+--------+------+--------+------+--------------+
6 rows in set (0.00 sec)

1.5、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和右边都有的结果
#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

mysql> 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;
+------+-------+--------+------+--------+------+--------------+
| id   | name  | sex    | age  | dep_id | id   | name         |
+------+-------+--------+------+--------+------+--------------+
|    1 | tom   | male   |   18 |    200 |  200 | 技术         |
|    5 | lili  | male   |   18 |    200 |  200 | 技术         |
|    2 | mike  | female |   48 |    201 |  201 | 人力资源     |
|    3 | jack  | male   |   38 |    201 |  201 | 人力资源     |
|    4 | lucy  | female |   28 |    202 |  202 | 销售         |
|    6 | alice | female |   18 |    204 | NULL | NULL         |
|    1 | tom   | male   |   18 |    200 |  200 | 技术         |
|    2 | mike  | female |   48 |    201 |  201 | 人力资源     |
|    3 | jack  | male   |   38 |    201 |  201 | 人力资源     |
|    4 | lucy  | female |   28 |    202 |  202 | 销售         |
|    5 | lili  | male   |   18 |    200 |  200 | 技术         |
| NULL | NULL  | NULL   | NULL |   NULL |  203 | 运营         |
+------+-------+--------+------+--------+------+--------------+
12 rows in set (0.00 sec)


mysql> mysql> 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;
+------+-------+--------+------+--------+------+--------------+
| id   | name  | sex    | age  | dep_id | id   | name         |
+------+-------+--------+------+--------+------+--------------+
|    1 | tom   | male   |   18 |    200 |  200 | 技术         |
|    5 | lili  | male   |   18 |    200 |  200 | 技术         |
|    2 | mike  | female |   48 |    201 |  201 | 人力资源     |
|    3 | jack  | male   |   38 |    201 |  201 | 人力资源     |
|    4 | lucy  | female |   28 |    202 |  202 | 销售         |
|    6 | alice | female |   18 |    204 | NULL | NULL         |
| NULL | NULL  | NULL   | NULL |   NULL |  203 | 运营         |
+------+-------+--------+------+--------+------+--------------+
7 rows in set (0.06 sec)

#注意 union与union all的区别：union会去掉相同的纪录

二、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;

三、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

3.1、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);
+------+--------------+
| id   | name         |
+------+--------------+
|  201 | 人力资源     |
|  202 | 销售         |
+------+--------------+

# 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');
+------+
| name |
+------+
| tom  |
| lili |
+------+

#查看不足1人的部门名
select name from department
    where id not in 
        (select dep_id from employee group by dep_id);

或
mysql> select name from department 
    where id not in 
       (select dep_id from employee);
+--------+
| name   |
+--------+
| 运营   |
+--------+
1 row in set (0.00 sec)

3.2、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 employee where age > (select avg(age) from employee);
+------+------+
| name | age  |
+------+------+
| mike |   48 |
| jack |   38 |
+------+------+

#查询大于部门内平均年龄的员工名、年龄
思路：
      （1）先对员工表(employee)中的人员分组(group by)，查询出dep_id以及平均年龄。
       (2)将查出的结果作为临时表,再对根据临时表的dep_id和employee的dep_id作为筛选条件将employee表和临时表进行内连接。
       (3)最后再将employee员工的年龄是大于平均年龄的员工名字和年龄筛选。

mysql> select name,age from employee as t1
    -> inner join
    -> (select dep_id, avg(age) as avg_age from employee group by dep_id) as t2
    -> on t1.dep_id=t2.dep_id
    -> where t1.age>t2.avg_age;
+------+------+
| name | age  |
+------+------+
| mike |   48 |
+------+------+
1 row in set (0.07 sec)

3.3、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=200，True
mysql> select * from employee  where exists (select id from department where id=200);
+----+-------+--------+------+--------+
| id | name  | sex    | age  | dep_id |
+----+-------+--------+------+--------+
|  1 | tom   | male   |   18 |    200 |
|  2 | mike  | female |   48 |    201 |
|  3 | jack  | male   |   38 |    201 |
|  4 | lucy  | female |   28 |    202 |
|  5 | lili  | male   |   18 |    200 |
|  6 | alice | female |   18 |    204 |
+----+-------+--------+------+--------+

#department表中不存在dept_id=204，False
mysql> select * from employee  where exists (select id from department where id=204);
Empty set (0.00 sec)