多表查询：
http://www.cnblogs.com/linhaifeng/articles/7267596.html
1.多表连接查询
2.符合条件连接查询
3.子查询
一、准备表记录
1 # 建表 2 create table department( 3 id int, 4 name varchar(20) 5 ); 6 create table employee( 7 id int primary key auto_increment, 8 name varchar(20), 9 sex enum('male','female') not null default 'male', 10 age int, 11 dep_id int 12 ); 13 # 插入数据 14 insert into department values 15 (200,'技术'), 16 (201,'人力资源'), 17 (202,'销售'), 18 (203,'运营'); 19 insert into employee(name,sex,age,dep_id) values 20 ('egon','male',18,200), 21 ('alex','female',48,201), 22 ('wupeiqi','male',38,201), 23 ('yuanhao','female',28,202), 24 ('liwenzhou','male',18,200), 25 ('jingliyang','female',18,204); 26 # 查看表结构和数据 27 mysql> desc department; 28 +-------+-------------+------+-----+---------+-------+ 29 | Field | Type | Null | Key | Default | Extra | 30 +-------+-------------+------+-----+---------+-------+ 31 | id | int(11) | YES | | NULL | | 32 | name | varchar(20) | YES | | NULL | | 33 +-------+-------------+------+-----+---------+-------+ 34 2 rows in set (0.00 sec) 35 36 mysql> desc employee; 37 +--------+-----------------------+------+-----+---------+----------------+ 38 | Field | Type | Null | Key | Default | Extra | 39 +--------+-----------------------+------+-----+---------+----------------+ 40 | id | int(11) | NO | PRI | NULL | auto_increment | 41 | name | varchar(20) | YES | | NULL | | 42 | sex | enum('male','female') | NO | | male | | 43 | age | int(11) | YES | | NULL | | 44 | dep_id | int(11) | YES | | NULL | | 45 +--------+-----------------------+------+-----+---------+----------------+ 46 5 rows in set (0.00 sec) 47 48 mysql> select * from department; 49 +------+--------------+ 50 | id | name | 51 +------+--------------+ 52 | 200 | 技术 | 53 | 201 | 人力资源 | 54 | 202 | 销售 | 55 | 203 | 运营 | 56 +------+--------------+ 57 4 rows in set (0.00 sec) 58 59 mysql> select * from employee; 60 +----+------------+--------+------+--------+ 61 | id | name | sex | age | dep_id | 62 +----+------------+--------+------+--------+ 63 | 1 | egon | male | 18 | 200 | 64 | 2 | alex | female | 48 | 201 | 65 | 3 | wupeiqi | male | 38 | 201 | 66 | 4 | yuanhao | female | 28 | 202 | 67 | 5 | liwenzhou | male | 18 | 200 | 68 | 6 | jingliyang | female | 18 | 204 | 69 +----+------------+--------+------+--------+ 70 6 rows in set (0.00 sec)
二、多表连接查询
#重点：外连接语法
SELECT 字段列表
FROM 表1 INNER|LEFT|RIGHT JOIN 表2
ON 表1.字段 = 表2.字段;
1.交叉连接：不适用任何匹配条件。生成笛卡尔积
1 mysql> select * from employee,department; 2 +----+------------+--------+------+--------+------+--------------+ 3 | id | name | sex | age | dep_id | id | name | 4 +----+------------+--------+------+--------+------+--------------+ 5 | 1 | egon | male | 18 | 200 | 200 | 技术 | 6 | 1 | egon | male | 18 | 200 | 201 | 人力资源 | 7 | 1 | egon | male | 18 | 200 | 202 | 销售 | 8 | 1 | egon | male | 18 | 200 | 203 | 运营 | 9 | 2 | alex | female | 48 | 201 | 200 | 技术 | 10 | 2 | alex | female | 48 | 201 | 201 | 人力资源 | 11 | 2 | alex | female | 48 | 201 | 202 | 销售 | 12 | 2 | alex | female | 48 | 201 | 203 | 运营 | 13 | 3 | wupeiqi | male | 38 | 201 | 200 | 技术 | 14 | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | 15 | 3 | wupeiqi | male | 38 | 201 | 202 | 销售 | 16 | 3 | wupeiqi | male | 38 | 201 | 203 | 运营 | 17 | 4 | yuanhao | female | 28 | 202 | 200 | 技术 | 18 | 4 | yuanhao | female | 28 | 202 | 201 | 人力资源 | 19 | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | 20 | 4 | yuanhao | female | 28 | 202 | 203 | 运营 | 21 | 5 | liwenzhou | male | 18 | 200 | 200 | 技术 | 22 | 5 | liwenzhou | male | 18 | 200 | 201 | 人力资源 | 23 | 5 | liwenzhou | male | 18 | 200 | 202 | 销售 | 24 | 5 | liwenzhou | male | 18 | 200 | 203 | 运营 | 25 | 6 | jingliyang | female | 18 | 204 | 200 | 技术 | 26 | 6 | jingliyang | female | 18 | 204 | 201 | 人力资源 | 27 | 6 | jingliyang | female | 18 | 204 | 202 | 销售 | 28 | 6 | jingliyang | female | 18 | 204 | 203 | 运营 | 29 +----+------------+--------+------+--------+------+--------------+
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
1 mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id; 2 +----+-----------+------+--------+--------------+ 3 | id | name | age | sex | name | 4 +----+-----------+------+--------+--------------+ 5 | 1 | egon | 18 | male | 技术 | 6 | 2 | alex | 48 | female | 人力资源 | 7 | 3 | wupeiqi | 38 | male | 人力资源 | 8 | 4 | yuanhao | 28 | female | 销售 | 9 | 5 | liwenzhou | 18 | male | 技术 | 10 +----+-----------+------+--------+--------------+ 11 12 #上述sql等同于 13 mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id; 14 15 内连接
3.外连接之左连接：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
1 mysql> select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id; 2 +----+------------+--------------+ 3 | id | name | depart_name | 4 +----+------------+--------------+ 5 | 1 | egon | 技术 | 6 | 5 | liwenzhou | 技术 | 7 | 2 | alex | 人力资源 | 8 | 3 | wupeiqi | 人力资源 | 9 | 4 | yuanhao | 销售 | 10 | 6 | jingliyang | NULL | 11 +----+------------+--------------+
4.外连接之右连接：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
1 mysql> select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id; 2 +------+-----------+--------------+ 3 | id | name | depart_name | 4 +------+-----------+--------------+ 5 | 1 | egon | 技术 | 6 | 2 | alex | 人力资源 | 7 | 3 | wupeiqi | 人力资源 | 8 | 4 | yuanhao | 销售 | 9 | 5 | liwenzhou | 技术 | 10 | NULL | NULL | 运营 | 11 +------+-----------+--------------+
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
全外连接三、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 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;
select employee.id,employee.name,employee.age,department.name
from employee
inner join department on employee.dep_id=department.id
where employee.age > 25 order 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：= 、 !=、> 、<等
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 in
(select dep_id from employee group by dep_id having count(id) <=1);
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) from emp);
+---------+------+
| name | age |
+---------+------+
| alex | 48 |
| wupeiqi | 38 |
+---------+------+
rows in set (0.00 sec)
#查询大于部门内平均年龄的员工名、年龄
select t1.name,t1.age from emp t1
inner join
(select dep_id,avg(age) avg_age from emp 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 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
+----+------------+--------+------+--------+
#department表中存在dept_id=204，False
mysql> select * from employee
-> where exists
-> (select id from department where id=204);
Empty set (0.00 sec)
4.练习
#查询每个部门最新入职的那位员工
#准备表和记录
company.employee
员工id id int
姓名 emp_name varchar
性别 sex enum
年龄 age int
入职日期 hire_date date
岗位 post varchar
职位描述 post_comment varchar
薪水 salary double
办公室 office int
部门编号 depart_id int
1 #创建表 2 create table employee( 3 id int not null unique auto_increment, 4 name varchar(20) not null, 5 sex enum('male','female') not null default 'male', #大部分是男的 6 age int(3) unsigned not null default 28, 7 hire_date date not null, 8 post varchar(50), 9 post_comment varchar(100), 10 salary double(15,2), 11 office int, #一个部门一个屋子 12 depart_id int 13 ); 14 15 #查看表结构 16 mysql> desc employee; 17 +--------------+-----------------------+------+-----+---------+----------------+ 18 | Field | Type | Null | Key | Default | Extra | 19 +--------------+-----------------------+------+-----+---------+----------------+ 20 | id | int(11) | NO | PRI | NULL | auto_increment | 21 | name | varchar(20) | NO | | NULL | | 22 | sex | enum('male','female') | NO | | male | | 23 | age | int(3) unsigned | NO | | 28 | | 24 | hire_date | date | NO | | NULL | | 25 | post | varchar(50) | YES | | NULL | | 26 | post_comment | varchar(100) | YES | | NULL | | 27 | salary | double(15,2) | YES | | NULL | | 28 | office | int(11) | YES | | NULL | | 29 | depart_id | int(11) | YES | | NULL | | 30 +--------------+-----------------------+------+-----+---------+----------------+ 31 32 #插入记录 33 #三个部门：教学，销售，运营 34 insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values 35 ('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 36 ('alex','male',78,'20150302','teacher',1000000.31,401,1), 37 ('wupeiqi','male',81,'20130305','teacher',8300,401,1), 38 ('yuanhao','male',73,'20140701','teacher',3500,401,1), 39 ('liwenzhou','male',28,'20121101','teacher',2100,401,1), 40 ('jingliyang','female',18,'20110211','teacher',9000,401,1), 41 ('jinxin','male',18,'19000301','teacher',30000,401,1), 42 ('成龙','male',48,'20101111','teacher',10000,401,1), 43 44 ('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门 45 ('丫丫','female',38,'20101101','sale',2000.35,402,2), 46 ('丁丁','female',18,'20110312','sale',1000.37,402,2), 47 ('星星','female',18,'20160513','sale',3000.29,402,2), 48 ('格格','female',28,'20170127','sale',4000.33,402,2), 49 50 ('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门 51 ('程咬金','male',18,'19970312','operation',20000,403,3), 52 ('程咬银','female',18,'20130311','operation',19000,403,3), 53 ('程咬铜','male',18,'20150411','operation',18000,403,3), 54 ('程咬铁','female',18,'20140512','operation',17000,403,3) 55 ; 56 57 #ps：如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbk 58 59 60 创建表 插入数据
# 答案一：
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;
# 答案二：
mysql> select (select t2.name from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post;
+---------------------------------------------------------------------------------------+
| (select t2.name from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) |
+---------------------------------------------------------------------------------------+
| 张野 |
| 格格 |
| alex |
| egon |
+---------------------------------------------------------------------------------------+
rows in set (0.00 sec)
mysql> select (select t2.id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post;
+-------------------------------------------------------------------------------------+
| (select t2.id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) |
+-------------------------------------------------------------------------------------+
| 14 |
| 13 |
| 2 |
| 1 |
+-------------------------------------------------------------------------------------+
rows in set (0.00 sec)
#正确答案
mysql> select t3.name,t3.post,t3.hire_date from emp as t3 where id in (select (select id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post);
+--------+-----------------------------------------+------------+
| name | post | hire_date |
+--------+-----------------------------------------+------------+
| egon | 老男孩驻沙河办事处外交大使 | 2017-03-01 |
| alex | teacher | 2015-03-02 |
| 格格 | sale | 2017-01-27 |
| 张野 | operation | 2016-03-11 |
+--------+-----------------------------------------+------------+
rows in set (0.00 sec)
注：答案一为正确答案，答案二中的limit 1有问题（每个部门可能有>1个为同一时间入职的新员工），我只是想用该例子来说明可以在select后使用子查询
可以基于上述方法解决：比如某网站在全国各个市都有站点，每个站点一条数据，想取每个省下最新的那一条市的网站质量信息