day--39-MySQL的多表查询

                                    多表查询
一：表的基本介绍

    可以参考：https://www.cnblogs.com/cdf-opensource-007/p/6517627.html
    建立一个员工表信息表和一个部门表，每个员工都对应在哪个部门。因此这两张表具有一定关系。需要将两个表格进行查询，找到一个
    员工所对应的所属部门。

01：建立一个员工表：employee 和一个部门表departmentemployee表：

　　mysql> 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
        -> );
    Query OK, 0 rows affected (0.18 sec)

    department表：
    mysql> create table department(
        -> id int,
        -> name varchar(20)
        -> );
    Query OK, 0 rows affected (0.12 sec)

 02：给两张表加入信息。
 employee表：

mysql> insert into employee(name,sex,age,dep_id) values
        -> ("egon","male",18,200),
        -> ("alex","female",48,201),
        -> ("wupeiqi","male",38,210),
        -> ("yuanhao","female",28,202),
        -> ("liwenzhou","female",28,202),
        -> ("jingwen","female",18,204);
    Query OK, 6 rows affected (0.05 sec)
    Records: 6  Duplicates: 0  Warnings: 0

　查看employee的结构：

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 (0.00 sec)

查看employee的信息：

mysql> select * from employee;
    +----+-----------+--------+------+--------+
    | id | name      | sex    | age  | dep_id |
    +----+-----------+--------+------+--------+
    |  1 | egon      | male   |   18 |    200 |
    |  2 | alex      | female |   48 |    201 |
    |  3 | wupeiqi   | male   |   38 |    210 |
    |  4 | yuanhao   | female |   28 |    202 |
    |  5 | liwenzhou | female |   28 |    202 |
    |  6 | jingwen   | female |   18 |    204 |
    +----+-----------+--------+------+--------+
    6 rows in set (0.00 sec)

department表：

mysql> insert into department values
        -> (200,"技术"),
        -> (201,"人力资源"),
        -> (202,"销售"),
        -> (203,"运营");
    Query OK, 4 rows affected (0.04 sec)
    Records: 4  Duplicates: 0  Warnings: 0

    department的结构：
    mysql> desc department;
    +-------+-------------+------+-----+---------+-------+
    | Field | Type        | Null | Key | Default | Extra |
    +-------+-------------+------+-----+---------+-------+
    | id    | int(11)     | YES  |     | NULL    |       |
    | name  | varchar(20) | YES  |     | NULL    |       |
    +-------+-------------+------+-----+---------+-------+
    2 rows in set (0.00 sec)

 department的信息：

mysql> select * from department;
    +------+--------------+
    | id   | name         |
    +------+--------------+
    |  200 | 技术         |
    |  201 | 人力资源     |
    |  202 | 销售         |
    |  203 | 运营         |
    +------+--------------+
    4 rows in set (0.00 sec)

二：多表查询（以employee和department为例）

#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；

01:交叉链接：生成笛卡儿积：

　　

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 |    210 |  200 | 技术         |
    |  3 | wupeiqi   | male   |   38 |    210 |  201 | 人力资源     |
    |  3 | wupeiqi   | male   |   38 |    210 |  202 | 销售         |
    |  3 | wupeiqi   | male   |   38 |    210 |  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 | female |   28 |    202 |  200 | 技术         |
    |  5 | liwenzhou | female |   28 |    202 |  201 | 人力资源     |
    |  5 | liwenzhou | female |   28 |    202 |  202 | 销售         |
    |  5 | liwenzhou | female |   28 |    202 |  203 | 运营         |
    |  6 | jingwen   | female |   18 |    204 |  200 | 技术         |
    |  6 | jingwen   | female |   18 |    204 |  201 | 人力资源     |
    |  6 | jingwen   | female |   18 |    204 |  202 | 销售         |
    |  6 | jingwen   | female |   18 |    204 |  203 | 运营         |
    +----+-----------+--------+------+--------+------+--------------+
    24 rows in set (0.00 sec)
    这种显示是将所有列表排序都生成了，我们要找的排序肯定会在这个笛卡儿积表格中，但是不利于分析。

02：内链接，只链接匹配的行。

　　

mysql> select * from employee,department where department.id=employee.dep_id;  #where条件分析。
    +----+-----------+--------+------+--------+------+--------------+
    | 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     |
    |  4 | yuanhao   | female |   28 |    202 |  202 | 销售         |
    |  5 | liwenzhou | female |   28 |    202 |  202 | 销售         |
    +----+-----------+--------+------+--------+------+--------------+
    4 rows in set (0.00 sec)

#再看一个需求，我要查出技术部的员工的名字

　　mysql> select name from employee,department where employee.dep_id=department.id and department.name='技术';
    ERROR 1052 (23000): Column 'name' in field list is ambiguous
    #上面直接就报错了，因为select后面直接写的name，在两个表合并起来的表中，是有两个name字段的，直接写name是不行的，要加上表名，再看：
    mysql> select employee.name from employee,department where employee.dep_id=department.id and department.name="技术";
    +------+
    | name |
    +------+
    | egon |
    +------+
    1 row in set (0.00 sec)

03:外链接之左链接：优先显示左边表的全部记录

　　#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：在内链接的基础上增加左边有而右边没有的结果
        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         |
            |  2 | alex      | 人力资源     |
            |  4 | yuanhao   | 销售         |
            |  5 | liwenzhou | 销售         |
            |  3 | wupeiqi   | NULL         |
            |  6 | jingwen   | NULL         |
            +----+-----------+--------------+
            6 rows in set (0.00 sec)

04:外链接之右链接：优先显示右边表的全部记录

　　#以右表为准，即找出所有员工信息，当然包括没有部门的员工
    #本质就是：在内链接的基础上增加右边有而左边没有的结果
    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     |
    |    4 | yuanhao   | 销售         |
    |    5 | liwenzhou | 销售         |
    | NULL | NULL      | 运营         |
    +------+-----------+--------------+
    5 rows in set (0.00 sec)

05:全外链接：显示左右两个表的全部内容
    001：union显示的左右两边的数据，并将重复的数据去重。

　　　　　　

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         |
    |    2 | alex      | female |   48 |    201 |  201 | 人力资源     |
    |    4 | yuanhao   | female |   28 |    202 |  202 | 销售         |
    |    5 | liwenzhou | female |   28 |    202 |  202 | 销售         |
    |    3 | wupeiqi   | male   |   38 |    210 | NULL | NULL         |
    |    6 | jingwen   | female |   18 |    204 | NULL | NULL         |
    | NULL | NULL      | NULL   | NULL |   NULL |  203 | 运营         |
    +------+-----------+--------+------+--------+------+--------------+
    7 rows in set (0.06 sec)

    002：union all  显示的是左右两边表的数据，并不会去重

　　　　

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 | egon      | male   |   18 |    200 |  200 | 技术         |
    |    2 | alex      | female |   48 |    201 |  201 | 人力资源     |
    |    4 | yuanhao   | female |   28 |    202 |  202 | 销售         |
    |    5 | liwenzhou | female |   28 |    202 |  202 | 销售         |
    |    3 | wupeiqi   | male   |   38 |    210 | NULL | NULL         |
    |    6 | jingwen   | female |   18 |    204 | NULL | NULL         |
    |    1 | egon      | male   |   18 |    200 |  200 | 技术         |
    |    2 | alex      | female |   48 |    201 |  201 | 人力资源     |
    |    4 | yuanhao   | female |   28 |    202 |  202 | 销售         |
    |    5 | liwenzhou | female |   28 |    202 |  202 | 销售         |
    | NULL | NULL      | NULL   | NULL |   NULL |  203 | 运营         |
    +------+-----------+--------+------+--------+------+--------------+
    11 rows in set (0.00 sec)

三：符合条件接连的查询
1、

#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
    mysql> select employee.name,department.name from employee inner join department
        -> on employee.dep_id=department.id
        -> where age>25;
    +-----------+--------------+
    | name      | name         |
    +-----------+--------------+
    | alex      | 人力资源     |
    | yuanhao   | 销售         |
    | liwenzhou | 销售         |
    +-----------+--------------+
    3 rows in set (0.00 sec)

2、#实例：以内连接的方式查询employee和department表，并且以age字段的生序方式显示；

　　

实例01：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
        mysql> select employee.name,department.name from employee inner join department
            -> on employee.dep_id=department.id  where age>25
            -> order by age;    #(order by  排序是默认的升序的，升序order by asc)
        +-----------+--------------+
        | name      | name         |
        +-----------+--------------+
        | yuanhao   | 销售         |
        | liwenzhou | 销售         |
        | alex      | 人力资源     |
        +-----------+--------------+
        3 rows in set (0.00 sec)

实例一

　　

实例02：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
        mysql> 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;
        +----+-----------+------+--------------+
        | id | name      | age  | name         |
        +----+-----------+------+--------------+
        |  5 | liwenzhou |   28 | 销售         |
        |  4 | yuanhao   |   28 | 销售         |
        |  2 | alex      |   48 | 人力资源     |
        +----+-----------+------+--------------+
        3 rows in set (0.00 sec)

实例二

四：子查询

解释：子查询就是将一个查询结果用括号括起来，交给另外一个sql语句，作为它的一个查询语句来进行操作。
        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

  1: 带in关键字的的字查询：

 mysql> 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         |
        +------+--------------+
        2 rows in set (0.05 sec)

    总结：子查询的思路和解决问题一样，先解决一个然后拿着这个的结果再去解决另外一个问题，连表的思路是先将两个表关联在一起，
    然后在进行group by啊过滤啊等等操作，两者的思路是不一样的.

　　

01:#查询员工平均年龄在25岁以上的部门名，可以用连表，也可以用子查询，
        mysql> select department.name from department inner join employee on department.id=employee.dep_id
            -> group by department.name having avg(age)>25;
        +--------------+
        | name         |
        +--------------+
        | 人力资源     |
        | 销售         |
        +--------------+

案例01

　　

02:#查看技术部员工姓名
        mysql> select name from employee where dep_id in (select id from department where name="技术");
        +------+
        | name |
        +------+
        | egon |
        +------+
        1 row in set (0.00 sec)

案例02

　　

03:#查看不足1人的部门名(子查询得到的是有人的部门id)
        mysql> select id,name from department where id not in (select distinct dep_id from employee);
        +------+--------+
        | id   | name   |
        +------+--------+
        |  203 | 运营   |
        +------+--------+
        1 row in set (0.00 sec)

案例03

2: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  |
    +---------+------+
    | alex    |   48 |
    | wupeiqi |   38 |
    +---------+------+
    2 rows in set (0.00 sec)

  3：带exists(存在）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。还可以写not exists，和exists的效果就是反的

    01：employee表中不存在id为200　　　　

　　 mysql> select * from employee where exists (select id from employee where id=500);
    Empty set (0.00 sec)　

    02：employee存在的情况：

　　#只有当括号里面的条件成立了，select * from employee  才会执行
    mysql> select * from employee where exists (select dep_id from employee where dep_id=200);
    +----+-----------+--------+------+--------+
    | id | name      | sex    | age  | dep_id |
    +----+-----------+--------+------+--------+
    |  1 | egon      | male   |   18 |    200 |
    |  2 | alex      | female |   48 |    201 |
    |  3 | wupeiqi   | male   |   38 |    210 |
    |  4 | yuanhao   | female |   28 |    202 |
    |  5 | liwenzhou | female |   28 |    202 |
    |  6 | jingwen   | female |   18 |    204 |
    +----+-----------+--------+------+--------+
    6 rows in set (0.00 sec)