一 多表连接查询

#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;

#建表

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),

('liwenzhou','male',18,200),

('jingliyang','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 | |

+-------+-------------+------+-----+---------+-------+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 | |

+--------+-----------------------+------+-----+---------+----------------+mysql> select * fromdepartment;+------+--------------+

| id | name |

+------+--------------+

| 200 | 技术 |

| 201 | 人力资源 |

| 202 | 销售 |

| 203 | 运营 |

+------+--------------+mysql> select * fromemployee;+----+------------+--------+------+--------+

| 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 | liwenzhou | male | 18 | 200 |

| 6 | jingliyang | female | 18 | 204 |

+----+------------+--------+------+--------+表department与employee

准备好表

1 交叉连接：不适用任何匹配条件。生成笛卡尔积

2 内连接：只连接匹配的行

#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正确的结果#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 | gouge | 18 | male | 技术 |

| 2 | 华仔 | 48 | female | 人力资源 |

| 3 | 老王 | 38 | male | 人力资源 |

| 4 | 刘哥 | 28 | female | 销售 |

| 5 | jjc | 18 | male | 技术 |

+----+-----------+------+--------+--------------+

#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;

3 外链接之左连接：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 | gouge | 技术 |

| 5 | 小明 | 技术 |

| 2 | 老王 | 人力资源 |

| 3 | 刘哥 | 人力资源 |

| 4 | jjc | 销售 |

| 6 | 黑人 | NULL |

+----+------------+--------------+

4 外链接之右连接：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 | gouge | 技术 |

| 2 | laowang | 人力资源 |

| 3 | jjc | 人力资源 |

| 4 | 刘哥 | 销售 |

| 5 | 文文 | 技术 |

| NULL | NULL | 运营 |

+------+-----------+--------------+

5 全外连接：显示左右两个表全部记录

全外连接：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

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

;#查看结果

+------+------------+--------+------+--------+------+--------------+

| id | name | sex | age | dep_id | id | name |

+------+------------+--------+------+--------+------+--------------+

| 1 | gouge | male | 18 | 200 | 200 | 技术 |

| 5 | jjc | male | 18 | 200 | 200 | 技术 |

| 2 | 老王 | female | 48 | 201 | 201 | 人力资源 |

| 3 | 华仔 | male | 38 | 201 | 201 | 人力资源 |

| 4 | 小明 | female | 28 | 202 | 202 | 销售 |

| 6 | jjz | female | 18 | 204 | NULL | NULL |

| NULL | NULL | NULL | NULL | NULL | 203 | 运营 |

+------+------------+--------+------+--------+------+--------------+

#注意 union与union all的区别：union会去掉相同的纪录

三 符合条件连接查询

#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

select employee.name,department.name fromemployee inner join department

on employee.dep_id=department.id

where age> 25;#示例2：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

select employee.id,employee.name,employee.age,department.name fromemployee,department

where employee.dep_id=department.idand age > 25order by age asc;

四 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：= 、 !=、> 、

1 带IN关键字的子查询

#查询平均年龄在25岁以上的部门名

select id,name fromdepartment

where idin(select dep_idfrom employee group by dep_id having avg(age) > 25);#查看技术部员工姓名

select name fromemployee

where dep_idin(select idfrom department where name='技术');#查看不足1人的部门名(子查询得到的是有人的部门id)

select name from department where id not in (select distinct dep_id from employee);

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

mysql> select name,age from emp where age > (select avg(age) fromemp);#查询大于部门内平均年龄的员工名、年龄

select t1.name,t1.age fromemp t1

inner join

(select dep_id,avg(age) avg_agefromemp group by dep_id) t2

on t1.dep_id=t2.dep_id

where t1.age> t2.avg_age;

3 带EXISTS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

#department表中存在dept_id=203，Ture

mysql> select * fromemployee->where exists-> (select id from department where id=200);

#department表中存在dept_id=205，False

mysql> select * from employee

-> where exists

-> (select id from department where id=204);

Empty set (0.00 sec)

一切的一切取决于sql语句的执行顺序！！