前言

简单的数据我们可以直接从一个表中获取，但在真实的项目中查询符合条件的数据通常需要牵扯到多张表，这就不得不使用多表查询。多表查询分为多表连接查询、符合条件链接查询、子查询。多表连接查询包括内连接、外连接、全连接。符合条件连接查询本质上是多表连接查询+过滤条件。子查询是将一个查询语句嵌套在另一个查询语句中，内层查询语句的查询结果作为外层查询语句的数据源。

准备

# 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

(1,'技术'),

(2,'财务'),

(3,'法律’);

# 此处省略员工表数据...

多表连接查询

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;

交叉连接

在介绍多表查询的时候，有必要先介绍下交叉连接，如下select * from employee, department;查询语句就是交叉连接查询，可以看出，同一个数据在在employee表和department表交叉连接之后产生了重复记录，其重复个数取决于department表的记录个数。所以最后交叉连接之后的记录个数是：count(employee) * count(department)，即笛卡尔积。通常情况下，笛卡尔积的结果在工作中无实际意义，我们需要在笛卡尔积的基础上进行筛选，找到employee.dep_id = department.id的那条记录。

mysql> select * from employee, department;

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

| id | name | age | sex | position | salary | dep_id | id | name |

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

| 1 | jack | 20 | male | lawyer | 888889 | 3 | 1 | 技术 |

| 1 | jack | 20 | male | lawyer | 888889 | 3 | 2 | 财务 |

| 1 | jack | 20 | male | lawyer | 888889 | 3 | 3 | 法律 |

| 2 | mark | 22 | male | lawyer | 888889 | 3 | 1 | 技术 |

| 2 | mark | 22 | male | lawyer | 888889 | 3 | 2 | 财务 |

| 2 | mark | 22 | male | lawyer | 888889 | 3 | 3 | 法律 |

| 3 | hank | 25 | male | lawyer | 7777.8 | 3 | 1 | 技术 |

| 3 | hank | 25 | male | lawyer | 7777.8 | 3 | 2 | 财务 |

| 3 | hank | 25 | male | lawyer | 7777.8 | 3 | 3 | 法律 |

| 4 | nick | 39 | male | lawyer | 4438890 | 3 | 1 | 技术 |

| 4 | nick | 39 | male | lawyer | 4438890 | 3 | 2 | 财务 |

| 4 | nick | 39 | male | lawyer | 4438890 | 3 | 3 | 法律 |

| 5 | jenny | 26 | female | lawyer | 10000.8 | 3 | 1 | 技术 |

| 5 | jenny | 26 | female | lawyer | 10000.8 | 3 | 2 | 财务 |

| 5 | jenny | 26 | female | lawyer | 10000.8 | 3 | 3 | 法律 |

| 6 | tony | 35 | male | RD | 100000000 | 1 | 1 | 技术 |

| 6 | tony | 35 | male | RD | 100000000 | 1 | 2 | 财务 |

| 6 | tony | 35 | male | RD | 100000000 | 1 | 3 | 法律 |

| 7 | emmy | 27 | female | RD | 9999 | 1 | 1 | 技术 |

| 7 | emmy | 27 | female | RD | 9999 | 1 | 2 | 财务 |

| 7 | emmy | 27 | female | RD | 9999 | 1 | 3 | 法律 |

| 8 | emmy | 23 | female | finance | 5000 | 2 | 1 | 技术 |

| 8 | emmy | 23 | female | finance | 5000 | 2 | 2 | 财务 |

| 8 | emmy | 23 | female | finance | 5000 | 2 | 3 | 法律 |

| 9 | lucy | 45 | female | finance | 10000 | 2 | 1 | 技术 |

| 9 | lucy | 45 | female | finance | 10000 | 2 | 2 | 财务 |

| 9 | lucy | 45 | female | finance | 10000 | 2 | 3 | 法律 |

| 10 | emmi | 20 | female | finance | 20000 | 2 | 1 | 技术 |

| 10 | emmi | 20 | female | finance | 20000 | 2 | 2 | 财务 |

| 10 | emmi | 20 | female | finance | 20000 | 2 | 3 | 法律 |

| 11 | james | 18 | male | NULL | 3000 | NULL | 1 | 技术 |

| 11 | james | 18 | male | NULL | 3000 | NULL | 2 | 财务 |

| 11 | james | 18 | male | NULL | 3000 | NULL | 3 | 法律 |

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

33 rows in set (0.00 sec)

内连接

内连接只会连接两张表匹配的行，即取交集。找两张表公共部分，相当于利用条件从笛卡尔积结果中筛选出了正确的结果

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 | jack | 20 | male | 法律 |

| 2 | mark | 22 | male | 法律 |

| 3 | hank | 25 | male | 法律 |

| 4 | nick | 39 | male | 法律 |

| 5 | jenny | 26 | female | 法律 |

| 6 | tony | 35 | male | 技术 |

| 7 | emmy | 27 | female | 技术 |

| 8 | emmy | 23 | female | 财务 |

| 9 | lucy | 45 | female | 财务 |

| 10 | emmi | 20 | female | 财务 |

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

10 rows in set (0.00 sec)

上述内连接查询语句等同于：

mysql> select employee.id,employee.name,employee.age,employee.sex,department.name

from employee,department

where employee.dep_id=department.id;

外连接

外连接分为左连接、右连接、全外连接

左连接

左连接在内连接的基础上优先显示左表全部记录。即左连接=内连接+左表未符合条件的记录

#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 |

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

| 6 | tony | 技术 |

| 7 | emmy | 技术 |

| 8 | emmy | 财务 |

| 9 | lucy | 财务 |

| 10 | emmi | 财务 |

| 1 | jack | 法律 |

| 2 | mark | 法律 |

| 3 | hank | 法律 |

| 4 | nick | 法律 |

| 5 | jenny | 法律 |

| 11 | james | NULL |

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

11 rows in set (0.00 sec)

右连接

又连接在内连接的基础上优先显示右表的内容。即右连接==内连接+右表未符合条件的记录

#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 | jack | 法律 |

| 2 | mark | 法律 |

| 3 | hank | 法律 |

| 4 | nick | 法律 |

| 5 | jenny | 法律 |

| 6 | tony | 技术 |

| 7 | emmy | 技术 |

| 8 | emmy | 财务 |

| 9 | lucy | 财务 |

| 10 | emmi | 财务 |

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

10 rows in set (0.00 sec)

可以发现，左表(employee表)的第11条记录没有被查询出来

全外连接

全外连接会在内连接查询的基础上显示左表和右表的全部记录

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 | age | sex | position | salary | dep_id | id | name |

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

| 6 | tony | 35 | male | RD | 100000000 | 1 | 1 | 技术 |

| 7 | emmy | 27 | female | RD | 9999 | 1 | 1 | 技术 |

| 8 | emmy | 23 | female | finance | 5000 | 2 | 2 | 财务 |

| 9 | lucy | 45 | female | finance | 10000 | 2 | 2 | 财务 |

| 10 | emmi | 20 | female | finance | 20000 | 2 | 2 | 财务 |

| 1 | jack | 20 | male | lawyer | 888889 | 3 | 3 | 法律 |

| 2 | mark | 22 | male | lawyer | 888889 | 3 | 3 | 法律 |

| 3 | hank | 25 | male | lawyer | 7777.8 | 3 | 3 | 法律 |

| 4 | nick | 39 | male | lawyer | 4438890 | 3 | 3 | 法律 |

| 5 | jenny | 26 | female | lawyer | 10000.8 | 3 | 3 | 法律 |

| 11 | james | 18 | male | NULL | 3000 | NULL | NULL | NULL |

| NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 人力 |

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

12 rows in set (0.00 sec)

符合条件链接查询

mysql> select employee.name, employee.age, department.name from employee inner join department

-> on employee.dep_id=department.id

-> where age > 30

-> order by age asc;

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

| name | age | name |

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

| tony | 35 | 技术 |

| nick | 39 | 法律 |

| lucy | 45 | 财务 |

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

3 rows in set (0.00 sec)

子查询

子查询是将一个查询语句的嵌套在另一个查询语句中

内层查询语句的查询结果作为外层查询语句的数据源

子查询中可以包含 IN、NOT IN、ANY、ALL、EXISTS和NOT EXISTS等关键字

注意被嵌套的查询语句需要用( )包裹

查询员工部门平均年龄大于30的部门信息

mysql> select * from department

-> where id in

-> (select dep_id from employee group by dep_id having avg(age) > 30);

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

| id | name |

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

| 1 | 技术 |

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

1 row in set (0.00 sec)

查询技术部员工姓名

mysql> select name from employee

-> where dep_id in

-> (select id from department where name="技术");

+------+

| name |

+------+

| tony |

| emmy |

+------+

2 rows in set (0.00 sec)

查询无员工的部门名(子查询得到的是所有人的部门id，需要disctinct去除)

mysql> select name from department

-> where id not in

-> (select distinct dep_id from employee);

带比较运算符的子查询

查询大于所有人平均年龄的员工名和年龄

mysql> select * from employee where age > (select avg(age) from employee);

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

| id | name | age | sex | position | salary | dep_id |

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

| 4 | nick | 39 | male | lawyer | 4438890 | 3 |

| 6 | tony | 35 | male | RD | 100000000 | 1 |

| 9 | lucy | 45 | female | finance | 10000 | 2 |

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

3 rows in set (0.00 sec)

不能这样：在前面没有group by的时后面不能使用分组函数

mysql> select * from employee where age > avg(age);

ERROR 1111 (HY000): Invalid use of group function

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

mysql> select * from employee

-> where exists

-> (select id from department where id=200);

MYSQL中可以把一个查询语句用括号括起来使用as起一个别名当做一个表使用

查询每个职位最新入职的员工

SELECT

*

FROM

emp AS t1

INNER JOIN (

SELECT

post,

max(hire_date) max_date

FROM

emp

GROUP BY

post

) AS t2 ON t1.post = t2.post

WHERE

t1.hire_date = t2.max_date;

查询语句关键字执行顺序

一个完整的mysql的查询语句如下：

SELECT DISTINCT

FROM

JOIN

ON

WHERE

GROUP BY

HAVING

ORDER BY

LIMIT

其关键字执行顺序如下：

(7) SELECT

(8) DISTINCT

(1) FROM

(3) JOIN

(2) ON

(4) WHERE

(5) GROUP BY

(6) HAVING

(9) ORDER BY

(10) LIMIT