Python进阶----多表查询(内连,左连,右连), 子查询(in,带比较运算符)

Python进阶----多表查询(内连,左连,右连), 子查询(in,带比较运算符)

一丶多表查询

    多表连接查询的应用场景:

​         连接是关系数据库模型的主要特点,也是区别于其他类型数据管理系的一个标志.

​         通常来说表与表之间的关系不必确定,也就时实体与实体之间的关系不紧密,检索数据时,使用连表操作增强灵活性.可以再连表查询时增加新的字段,为不同实体创建新的表.

    多表联查的基本语句:

# 多表连接查询语法
select  字段
	from 表1
	INNER|LEFT|RIGHT  join   表2
	on 表1.字段=表2.字段;
	# 条件过滤
	where 条件

	# 1.形成新的表
	# 2.灵活的操控两个表的所有字段
	# 3.提高效率

    交叉连接:

​            即笛卡尔积,将两个表所有的记录进行配对,数据大量冗余,没有实际意义

### 查询 employee 员工表 和 department 部门表的笛卡尔积
	# 数据大量冗余 ,没有实际意义
	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:

​            内连接是一种最常用的连接类型,两个表的字段满足的连接条件,只显示两张表中互相匹配的项,其他不匹配项的不显示.

### 将员工表(employee)与部门表(department)进行连接,  员工表通过字段dep_id(部门id) 与 部门表字段id(部门id)进行连接

    # 方式一 自连接 (特殊的内连接) 通过where 方式进行连接
        select * from employee e ,department d  where e.dep_id=d.id ;
        # 结果
        +----+-----------+--------+------+--------+------+--------------+
        | id | name      | sex    | age  | dep_id | id   | name         |
        +----+-----------+--------+------+--------+------+--------------+
        |  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 | liwenzhou | male   |   18 |    200 |  200 | 技术         |
        +----+-----------+--------+------+--------+------+--------------+

    # 方式二 通过内连接  ... inner join ... on方式连接
    	select * from employee e inner join department d on e.dep_id=d.id ;
    	# 结果
        +----+-----------+--------+------+--------+------+--------------+
        | id | name      | sex    | age  | dep_id | id   | name         |
        +----+-----------+--------+------+--------+------+--------------+
        |  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 | liwenzhou | male   |   18 |    200 |  200 | 技术         |
        +----+-----------+--------+------+--------+------+--------------+

    外连接:

​            左外连接:

​                语法:A LEFT join B on A.XX=B.XX;

                左表存显示所有,右表没有与左表匹配的则为 null.

            右外连接:

​                语法:A RIGHT join B on A.XX=B.XX;

​                右表存显示所有,左表没有与右表匹配的则为 null.

###  员工表和部门表
# 左外连接 : 与左表没有匹配的则显示空,左表完全显示
	select * from employee e left join department d on e.dep_id=d.id;
	# 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         |  #### 重点~~~
    +----+------------+--------+------+--------+------+--------------+

# 右外连接 : 与右表没有匹配的则显示空,右表完全显示
	select * from employee e right join department d on e.dep_id=d.id;
	# 结果:
	+------+-----------+--------+------+--------+------+--------------+
    | id   | name      | sex    | age  | dep_id | id   | name         |
    +------+-----------+--------+------+--------+------+--------------+
    |    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 | liwenzhou | male   |   18 |    200 |  200 | 技术         |
    | NULL | NULL      | NULL   | NULL |   NULL |  203 | 运营         | #### 重点~~
    +------+-----------+--------+------+--------+------+--------------+

    全连接:

            union 关键字

            mysql 不支持全外连接 full JOIN

​            mysql 使用 union 连接左连接和右连接，得到全连接

### union 全连接
	# 必须是 左连接 + 右连接
	select * from employee e left join department d on e.dep_id=d.id
	union
	select * from employee e right join department d on e.dep_id=d.id;
	# 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         | ### 重点