准备工作：准备两张表，部门表（department）、员工表（employee）
mysql> create database db2;
Query OK, 1 row affected (0.00 sec)
mysql> use db2;
Database changed
创建表
mysql> create table department(id int,name varchar(20));
Query OK, 0 rows affected (0.17 sec)
mysql> 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);
Query OK, 0 rows affected (0.16 sec)
插入数据
mysql> insert into department values(200,'技术'),(201,'人力资源'),(202,'销售'),(203,'运营');
Query OK, 4 rows affected (0.06 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> insert into employee(name,sex,age,dep_id) values('zhangsan','male',18,200),('lisi','female',48,201),('wangwu','male',38,201),('zhuliu','female',28,202),('chenqi','male',18,200),('wangba','female',18,204);
Query OK, 6 rows affected (0.01 sec)
Records: 6 Duplicates: 0 Warnings: 0
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 (0.00 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 (0.00 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 (0.00 sec)
mysql> select * from employee;
+----+----------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+----------+--------+------+--------+
| 1 | zhangsan | male | 18 | 200 |
| 2 | lisi | female | 48 | 201 |
| 3 | wangwu | male | 38 | 201 |
| 4 | zhuliu | female | 28 | 202 |
| 5 | chenqi | male | 18 | 200 |
| 6 | wangba | 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
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 | zhangsan | male | 18 | 200 | 200 | 技术 |
| 1 | zhangsan | male | 18 | 200 | 201 | 人力资源 |
| 1 | zhangsan | male | 18 | 200 | 202 | 销售 |
| 1 | zhangsan | male | 18 | 200 | 203 | 运营 |
| 2 | lisi | female | 48 | 201 | 200 | 技术 |
| 2 | lisi | female | 48 | 201 | 201 | 人力资源 |
| 2 | lisi | female | 48 | 201 | 202 | 销售 |
| 2 | lisi | female | 48 | 201 | 203 | 运营 |
| 3 | wangwu | male | 38 | 201 | 200 | 技术 |
| 3 | wangwu | male | 38 | 201 | 201 | 人力资源 |
| 3 | wangwu | male | 38 | 201 | 202 | 销售 |
| 3 | wangwu | male | 38 | 201 | 203 | 运营 |
| 4 | zhuliu | female | 28 | 202 | 200 | 技术 |
| 4 | zhuliu | female | 28 | 202 | 201 | 人力资源 |
| 4 | zhuliu | female | 28 | 202 | 202 | 销售 |
| 4 | zhuliu | female | 28 | 202 | 203 | 运营 |
| 5 | chenqi | male | 18 | 200 | 200 | 技术 |
| 5 | chenqi | male | 18 | 200 | 201 | 人力资源 |
| 5 | chenqi | male | 18 | 200 | 202 | 销售 |
| 5 | chenqi | male | 18 | 200 | 203 | 运营 |
| 6 | wangba | female | 18 | 204 | 200 | 技术 |
| 6 | wangba | female | 18 | 204 | 201 | 人力资源 |
| 6 | wangba | female | 18 | 204 | 202 | 销售 |
| 6 | wangba | female | 18 | 204 | 203 | 运营 |
+----+----------+--------+------+--------+------+--------------+
24 rows in set (0.11 sec)
（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 | zhangsan | 18 | male | 技术 |
| 2 | lisi | 48 | female | 人力资源 |
| 3 | wangwu | 38 | male | 人力资源 |
| 4 | zhuliu | 28 | female | 销售 |
| 5 | chenqi | 18 | male | 技术 |
+----+----------+------+--------+--------------+
5 rows in set (0.01 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;
+----+----------+------+--------+--------------+
| id | name | age | sex | name |
+----+----------+------+--------+--------------+
| 1 | zhangsan | 18 | male | 技术 |
| 2 | lisi | 48 | female | 人力资源 |
| 3 | wangwu | 38 | male | 人力资源 |
| 4 | zhuliu | 28 | female | 销售 |
| 5 | chenqi | 18 | male | 技术 |
+----+----------+------+--------+--------------+
5 rows in set (0.00 sec)
（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 | zhangsan | 技术 |
| 5 | chenqi | 技术 |
| 2 | lisi | 人力资源 |
| 3 | wangwu | 人力资源 |
| 4 | zhuliu | 销售 |
| 6 | wangba | 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 | zhangsan | 技术 |
| 2 | lisi | 人力资源 |
| 3 | wangwu | 人力资源 |
| 4 | zhuliu | 销售 |
| 5 | chenqi | 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
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 select * from employee right join department on employee.dep_id = department.id;
+------+----------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+------+----------+--------+------+--------+------+--------------+
| 1 | zhangsan | male | 18 | 200 | 200 | 技术 |
| 5 | chenqi | male | 18 | 200 | 200 | 技术 |
| 2 | lisi | female | 48 | 201 | 201 | 人力资源 |
| 3 | wangwu | male | 38 | 201 | 201 | 人力资源 |
| 4 | zhuliu | female | 28 | 202 | 202 | 销售 |
| 6 | wangba | female | 18 | 204 | NULL | NULL |
| NULL | NULL | NULL | NULL | NULL | 203 | 运营 |
+------+----------+--------+------+--------+------+--------------+
7 rows in set (0.00 sec)
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;
+------+----------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+------+----------+--------+------+--------+------+--------------+
| 1 | zhangsan | male | 18 | 200 | 200 | 技术 |
| 5 | chenqi | male | 18 | 200 | 200 | 技术 |
| 2 | lisi | female | 48 | 201 | 201 | 人力资源 |
| 3 | wangwu | male | 38 | 201 | 201 | 人力资源 |
| 4 | zhuliu | female | 28 | 202 | 202 | 销售 |
| 6 | wangba | female | 18 | 204 | NULL | NULL |
| 1 | zhangsan | male | 18 | 200 | 200 | 技术 |
| 2 | lisi | female | 48 | 201 | 201 | 人力资源 |
| 3 | wangwu | male | 38 | 201 | 201 | 人力资源 |
| 4 | zhuliu | female | 28 | 202 | 202 | 销售 |
| 5 | chenqi | male | 18 | 200 | 200 | 技术 |
| NULL | NULL | NULL | NULL | NULL | 203 | 运营 |
+------+----------+--------+------+--------+------+--------------+
12 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
mysql> select employee.name,department.name from employee inner join department on employee.dep_id = department.id where age > 25;
+--------+--------------+
| name | name |
+--------+--------------+
| lisi | 人力资源 |
| wangwu | 人力资源 |
| zhuliu | 销售 |
+--------+--------------+
3 rows in set (0.00 sec)
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
mysql> 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;
+----+--------+------+--------------+
| id | name | age | name |
+----+--------+------+--------------+
| 4 | zhuliu | 28 | 销售 |
| 3 | wangwu | 38 | 人力资源 |
| 2 | lisi | 48 | 人力资源 |
+----+--------+------+--------------+
3 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 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
mysql> select id,name from department where id in (select dep_id from employee group by dep_id having avg(age) > 25);
+------+--------------+
| id | name |
+------+--------------+
| 201 | 人力资源 |
| 202 | 销售 |
+------+--------------+
2 rows in set (0.00 sec)
查看技术部员工姓名
mysql> select name from employee where dep_id in (select id from department where name='技术');
+----------+
| name |
+----------+
| zhangsan |
| chenqi |
+----------+
2 rows in set (0.05 sec)
查看不足1人的部门名
mysql> select name from department where id not in (select dep_id from employee group by dep_id);
+--------+
| name |
+--------+
| 运营 |
+--------+
1 row in set (0.00 sec)
（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 |
+--------+------+
| lisi | 48 |
| wangwu | 38 |
+--------+------+
2 rows in set (0.00 sec)
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 |
+------+------+
| lisi | 48 |
+------+------+
1 row in set (0.00 sec)
(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 | zhangsan | male | 18 | 200 |
| 2 | lisi | female | 48 | 201 |
| 3 | wangwu | male | 38 | 201 |
| 4 | zhuliu | female | 28 | 202 |
| 5 | chenqi | male | 18 | 200 |
| 6 | wangba | female | 18 | 204 |
+----+----------+--------+------+--------+
6 rows in set (0.00 sec)
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
mysql> create table info(id int not null unique auto_increment,name varchar(20) not null,sex enum('male','female') nott null default 'male',age int(3) unsigned not null default 28,hire_date date not null,post varchar(50),post_comment vaarchar(100),salary double(15,2),office int, depart_id int);
Query OK, 0 rows affected (0.10 sec)
查看表结构
mysql> desc info;
+--------------+-----------------------+------+-----+---------+----------------+
| 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 | |
+--------------+-----------------------+------+-----+---------+----------------+
10 rows in set (0.00 sec)
插入记录
三个部门：教学，销售，运营
mysql> insert into info(name,sex,age,hire_date,post,salary,office,depart_id) values
('zhangsan','male',18,'20170301','公关部',7300.33,401,1),
('lisi','male',78,'20150302','teacher',1000000.31,401,1),
('wangwu','male',81,'20130305','teacher',8300,401,1),
('zhuliu','male',73,'20140701','teacher',3500,401,1),
('chenqi','male',28,'20121101','teacher',2100,401,1),
('wangba','female',18,'20110211','teacher',9000,401,1),
('tianjiu','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),
('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),
('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)
;
Query OK, 18 rows affected (0.00 sec)
Records: 18 Duplicates: 0 Warnings: 0
mysql> select t1.post,t1.name,t1.hire_date from info as t1 inner join (select post,max(hire_date) as new_date from info group by post) as t2 on t1.post=t2.post where t1.hire_date=t2.new_date;
+-----------+----------+------------+
| post | name | hire_date |
+-----------+----------+------------+
| 公关部 | zhangsan | 2017-03-01 |
| teacher | lisi | 2015-03-02 |
| sale | 格格 | 2017-01-27 |
| operation | 张野 | 2016-03-11 |
+-----------+----------+------------+
4 rows in set (0.01 sec)
