多表查询

本节重点：

• 多表连接查询
• 符合条件连接查询
• 子查询

　　

准备工作：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    |       | +-------+-------------+------+-----+---------+-------+ 2 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    |                | +--------+-----------------------+------+-----+---------+----------------+ 5 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         | +------+--------------+ 4 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 | +----+----------+--------+------+--------+ 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、多表连接查询

两张表的准备工作已完成，比如现在我要查询的员工信息以及该员工所在的部门。从该题中，我们看出既要查员工又要查该员工的部门，肯定要将两张表进行连接查询，多表连接查询。

重点：外链接语法

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;

（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（关于笛卡尔积的含义，大家百度自行补脑）。

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

#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;

（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 | 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

#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;   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

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 from employee 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 from employee,department     where employee.dep_id = department.id     and age > 25     order by age asc;

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

例子：

（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 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

#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。    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

#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 * 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，False mysql> select * from employee  where exists (select id from department where id=204); Empty set (0.00 sec)

小练习：

查询每个部门最新入职的那位员工

#创建表 create table employee( id int not null unique auto_increment, name varchar(20) not null, sex enum('male','female') not null default 'male', #大部分是男的 age int(3) unsigned not null default 28, hire_date date not null, post varchar(50), post_comment varchar(100), salary double(15,2), office int, #一个部门一个屋子 depart_id int );   #查看表结构 mysql> desc employee; +--------------+-----------------------+------+-----+---------+----------------+ | Field        | Type                  | Null | Key | Default | Extra          | +--------------+-----------------------+------+-----+---------+----------------+ | id           | int(11)               | NO   | PRI | NULL    | auto_increment | | name         | varchar(20)           | NO   |     | NULL    |                | | sex          | enum('male','female') | NO   |     | male    |                | | age          | int(3) unsigned       | NO   |     | 28      |                | | hire_date    | date                  | NO   |     | NULL    |                | | post         | varchar(50)           | YES  |     | NULL    |                | | post_comment | varchar(100)          | YES  |     | NULL    |                | | salary       | double(15,2)          | YES  |     | NULL    |                | | office       | int(11)               | YES  |     | NULL    |                | | depart_id    | int(11)               | YES  |     | NULL    |                | +--------------+-----------------------+------+-----+---------+----------------+  #插入记录 #三个部门：教学，销售，运营 insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values ('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部 ('alex','male',78,'20150302','teacher',1000000.31,401,1), ('wupeiqi','male',81,'20130305','teacher',8300,401,1), ('yuanhao','male',73,'20140701','teacher',3500,401,1), ('liwenzhou','male',28,'20121101','teacher',2100,401,1), ('jingliyang','female',18,'20110211','teacher',9000,401,1), ('jinxin','male',18,'19000301','teacher',30000,401,1), ('成龙','male',48,'20101111','teacher',10000,401,1),  ('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门 ('丫丫','female',38,'20101101','sale',2000.35,402,2), ('丁丁','female',18,'20110312','sale',1000.37,402,2), ('星星','female',18,'20160513','sale',3000.29,402,2), ('格格','female',28,'20170127','sale',4000.33,402,2),  ('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门 ('程咬金','male',18,'19970312','operation',20000,403,3), ('程咬银','female',18,'20130311','operation',19000,403,3), ('程咬铜','male',18,'20150411','operation',18000,403,3), ('程咬铁','female',18,'20140512','operation',17000,403,3) ;

select * from employee as t1 inner join (select post,max(hire_date) as new_date from employee group by post) as t2 on t1.post=t2.post where t1.hire_date=t2.new_date;

• 多表连接查询
• 符合条件连接查询
• 子查询

　　

准备工作：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    |       | +-------+-------------+------+-----+---------+-------+ 2 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    |                | +--------+-----------------------+------+-----+---------+----------------+ 5 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         | +------+--------------+ 4 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 | +----+----------+--------+------+--------+ 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、多表连接查询

两张表的准备工作已完成，比如现在我要查询的员工信息以及该员工所在的部门。从该题中，我们看出既要查员工又要查该员工的部门，肯定要将两张表进行连接查询，多表连接查询。

重点：外链接语法

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;

（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（关于笛卡尔积的含义，大家百度自行补脑）。

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

#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;

（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 | 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

#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;   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

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 from employee 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 from employee,department     where employee.dep_id = department.id     and age > 25     order by age asc;

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

例子：

（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 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

#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。    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

#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 * 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，False mysql> select * from employee  where exists (select id from department where id=204); Empty set (0.00 sec)

小练习：

查询每个部门最新入职的那位员工

#创建表 create table employee( id int not null unique auto_increment, name varchar(20) not null, sex enum('male','female') not null default 'male', #大部分是男的 age int(3) unsigned not null default 28, hire_date date not null, post varchar(50), post_comment varchar(100), salary double(15,2), office int, #一个部门一个屋子 depart_id int );   #查看表结构 mysql> desc employee; +--------------+-----------------------+------+-----+---------+----------------+ | Field        | Type                  | Null | Key | Default | Extra          | +--------------+-----------------------+------+-----+---------+----------------+ | id           | int(11)               | NO   | PRI | NULL    | auto_increment | | name         | varchar(20)           | NO   |     | NULL    |                | | sex          | enum('male','female') | NO   |     | male    |                | | age          | int(3) unsigned       | NO   |     | 28      |                | | hire_date    | date                  | NO   |     | NULL    |                | | post         | varchar(50)           | YES  |     | NULL    |                | | post_comment | varchar(100)          | YES  |     | NULL    |                | | salary       | double(15,2)          | YES  |     | NULL    |                | | office       | int(11)               | YES  |     | NULL    |                | | depart_id    | int(11)               | YES  |     | NULL    |                | +--------------+-----------------------+------+-----+---------+----------------+  #插入记录 #三个部门：教学，销售，运营 insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values ('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部 ('alex','male',78,'20150302','teacher',1000000.31,401,1), ('wupeiqi','male',81,'20130305','teacher',8300,401,1), ('yuanhao','male',73,'20140701','teacher',3500,401,1), ('liwenzhou','male',28,'20121101','teacher',2100,401,1), ('jingliyang','female',18,'20110211','teacher',9000,401,1), ('jinxin','male',18,'19000301','teacher',30000,401,1), ('成龙','male',48,'20101111','teacher',10000,401,1),  ('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门 ('丫丫','female',38,'20101101','sale',2000.35,402,2), ('丁丁','female',18,'20110312','sale',1000.37,402,2), ('星星','female',18,'20160513','sale',3000.29,402,2), ('格格','female',28,'20170127','sale',4000.33,402,2),  ('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门 ('程咬金','male',18,'19970312','operation',20000,403,3), ('程咬银','female',18,'20130311','operation',19000,403,3), ('程咬铜','male',18,'20150411','operation',18000,403,3), ('程咬铁','female',18,'20140512','operation',17000,403,3) ;

select * from employee as t1 inner join (select post,max(hire_date) as new_date from employee group by post) as t2 on t1.post=t2.post where t1.hire_date=t2.new_date;