本文实例讲述了MySQL多表查询。分享给大家供大家参考，具体如下：
准备工作：准备两张表，部门表（department）、员工表（employee）
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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); |
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#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); |
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# 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 | |+-------+-------------+------+-----+---------+-------+2 rows in set (0.19 sec) |
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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 (0.01 sec) |
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mysql> select * from department;+------+--------------+| id | name |+------+--------------+| 200 | 技术 || 201 | 人力资源 || 202 | 销售 || 203 | 运营 |+------+--------------+4 rows in set (0.02 sec) |
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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 |+----+----------+--------+------+--------+6 rows in set (0.00 sec) |
ps:观察两张表，发现department表中id=203部门在employee中没有对应的员工，发现employee中id=6的员工在department表中没有对应关系。
一多表链接查询
SELECT 字段列表
FROM 表1 INNER|LEFT|RIGHT JOIN 表2
ON 表1.字段 = 表2.字段;
（1）先看第一种情况交叉连接：不适用任何匹配条件。生成笛卡尔积.--->重复最多
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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 | 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 | |
（2）内连接：只连接匹配的行，以双方为基准
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#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) |
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#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
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#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) |
（4） 外链接之右连接：优先显示右表全部记录
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#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) |
（5） 全外连接：显示左右两个表全部记录(了解)
#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语法：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;
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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 (0.01 sec)#注意 union与union all的区别：union会去掉相同的纪录 |
二、符合条件连接查询
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
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select employee.name,department.name from employee inner join department on employee.dep_id = department.id where age > 25; |
三、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关键字的子查询
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#查询平均年龄在25岁以上的部门名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 name from employee where dep_id in (select id from department where name='技术');#查看不足1人的部门名select name from department where id not in (select dep_id from employee group by dep_id); |
（2）带比较运算符的子查询
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#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 |+---------+------+#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,再对根据临时表的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。
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mysql> 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 | |
(3)带EXISTS关键字的子查询
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#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，Turemysql> 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 |+----+----------+--------+------+--------+#department表中存在dept_id=205，Falsemysql> select * from employee where exists (select id from department where id=204);Empty set (0.00 sec) |
希望本文所述对大家MySQL数据库计有所帮助。
原文链接：https://www.cnblogs.com/mmyy-blog/p/9628548.html
