【MySQL】多表查询 -- 2019-08-08 20:38:59

多表链接查询

# 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;

笛卡尔积：

即交叉链接
不适用任何匹配条件，生成笛卡尔积（关于笛卡尔积的含义，请自行百度）

select * from employee, department;    # 语法

内链接 inner join

只链接匹配的行

# 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;
+----+---------+-----+--------+--------------+
| 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;

外链接之左链接 left join

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 | 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)

外链接之右链接 right join

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 | nvshen  | 技术         |
| NULL | NULL    | 运营         |
+------+---------+--------------+
6 rows in set (0.00 sec)

全外链接

显示左右两个表全部记录

# 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
 
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;
 
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 | 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)
 
# 注意 union与union all的区别：union会去掉相同的纪录

符合条件链接查询

# 示例1 以内链接的方式查询：找出年龄大于25岁的员工以及员工所在的部门
mysql> select employee.name,employee.age,department.name
    -> from employee inner join department
    -> on employee.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)
 
 
# 示例2 以内链接的方式查询：以age字段的升序方式显示
mysql> select employee.name,employee.age,department.name
    -> from employee inner join department
    -> on employee.dep_id = department.id
    -> order by age asc;    # 升序排序
+---------+-----+--------------+
| name    | age | name         |
+---------+-----+--------------+
| egon    |  18 | 技术         |
| nvshen  |  18 | 技术         |
| yuanhao |  28 | 销售         |
| wupeiqi |  38 | 人力资源     |
| alex    |  48 | 人力资源     |
+---------+-----+--------------+
5 rows in set (0.00 sec)

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、not exists 等关键字.
4. 还可以包含比较运算符：=、 !=、>、< 等.

示例1：带in关键字的子查询

# 查询平均年龄在25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 id,name from employee
    where dep_id in
        (select id from department where name="技术");
 
# 查无人的部门名
select name from department
    where id not in
        (select dep_id from employee);

示例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
select name,age from employee 
    where age > (select avg(age) from employee);
 
# 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。
 
select t1.name,t1.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 |
+------+-----+
| alex |  48 |
+------+-----+
1 row in set (0.00 sec)

示例3：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
 
# exists为True时：
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 |
+----+----------+--------+-----+--------+
6 rows in set (0.00 sec)
 
# exists为False时：
mysql> select * from employee where exists (select id from department where id=204);
Empty set (0.00 sec)

原文: http://106.13.73.98/__/26/

【MySQL】多表查询 -- 2019-08-08 20:38:59

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