# 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

 # 部门表
create table department(
id int primary key auto_increment,
name varchar(20) not null
);
# 员工表
create table employee(
id int primary key auto_increment,
name varchar(20) not null,
sex enum('male', 'female') not null default 'male',
age int not null,
dep_id int not null
);
# 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)
;
# 注意：
department表中id=203的部门在employee中没有对应的员工
employee表中id=6的员工在department表中没有对应的部门


# 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)












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