MySql

主要内容

• 1.表和表的关系
• 2.单表查询
• 3.多表查询

1.表和表的关系(外键的变种)

分析步骤：
#1、先站在左表的角度去找
是否左表的多条记录可以对应右表的一条记录，如果是，则证明左表的一个字段foreign key 右表一个字段（通常是id）

#2、再站在右表的角度去找
是否右表的多条记录可以对应左表的一条记录，如果是，则证明右表的一个字段foreign key 左表一个字段（通常是id）

#3、总结：
#多对一：
如果只有步骤1成立，则是左表多对一右表
如果只有步骤2成立，则是右表多对一左表

#多对多
如果步骤1和2同时成立，则证明这两张表时一个双向的多对一，即多对多,需要定义一个这两张表的关系表来专门存放二者的关系

#一对一:
如果1和2都不成立，而是左表的一条记录唯一对应右表的一条记录，反之亦然。这种情况很简单，就是在左表foreign key右表的基础上，将左表的外键字段设置成unique即可

3.多表查询

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
('egon','male',18,200),
('alex','female',48,201),
('wupeiqi','male',38,201),
('yuanhao','female',28,202),
('nvshen','male',18,200),
('xiaomage','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    |       |
+-------+-------------+------+-----+---------+-------+
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    |                |
+--------+-----------------------+------+-----+---------+----------------+
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         |
+------+--------------+
rows in set (0.02 sec)

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 | nvshen   | male   |   18 |    200 |
|  6 | xiaomage | female |   18 |    204 |
+----+----------+--------+------+--------+
rows in set (0.00 sec)

 (1)多表连接查询

笛卡尔积:

mysql> select * from employee2,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 | nvshen   | male   |   18 |    200 |  200 | 技术         |
|  5 | nvshen   | male   |   18 |    200 |  201 | 人力资源     |
|  5 | nvshen   | male   |   18 |    200 |  202 | 销售         |
|  5 | nvshen   | male   |   18 |    200 |  203 | 运营         |
|  6 | xiaomage | female |   18 |    204 |  200 | 技术         |
|  6 | xiaomage | female |   18 |    204 |  201 | 人力资源     |
|  6 | xiaomage | female |   18 |    204 |  202 | 销售         |
|  6 | xiaomage | female |   18 |    204 |  203 | 运营         |
+----+----------+--------+------+--------+------+--------------+
24 rows in set (0.11 sec)

#符合条件查询
mysql> select * from employee2,department where employee2.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     |
|  3 | wupeiqi | male   |   38 |    201 |  201 | 人力资源     |
|  4 | yuanhao | female |   28 |    202 |  202 | 销售         |
|  5 | nvshen  | male   |   18 |    200 |  200 | 技术         |
+----+---------+--------+------+--------+------+--------------+

(2) 内连接 (只获取匹配数据)

#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 * from employee2 inner join department on employee2.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     |
|  3 | wupeiqi | male   |   38 |    201 |  201 | 人力资源     |
|  4 | yuanhao | female |   28 |    202 |  202 | 销售         |
|  5 | nvshen  | male   |   18 |    200 |  200 | 技术         |
+----+---------+--------+------+--------+------+--------------+

(3)左连接:优先显示左表所有记录

mysql> select *from employee2 left join department on employee2.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         |
+----+----------+--------+------+--------+------+--------------+

(4)全外连接

mysql> select * from employee2 left join department on employee2.dep_id = department.id
    ->   union
    -> select * from employee2 right join department on employee2.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)

练习题:

#1.即找出年龄大于25岁的员工以及员工所在的部门
mysql> select department.name,employee2.name from employee2 inner join department on employee2.dep_id=department.id where age>25;
+--------------+---------+
| name         | name    |
+--------------+---------+
| 人力资源     | alex    |
| 人力资源     | wupeiqi |
| 销售         | yuanhao |
+--------------+---------+

(5)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

小练习:

# 查询大于部门内平均年龄的员工名、年龄
select name,age from employee2 inner join (select dep_id,avg(age) as av from employee2 
group by dep_id) as B on employee2.dep_id = B.dep_id where age > av;

#查询平均年龄在25岁以上的部门名
select * from department where id in (select dep_id from employee group by dep_id having avg(age) > 25);