SQL 内连接，左连接，左外连接，右连接，右外连接，全连接，交叉连接
内联接使用比较运算符根据每个表共有的列的值匹配两个表中的行。例如，检索 students 和 courses 表中学生标识号相同的所有行。
外联接。外联接可以是左向外联接、右向外联接或完整外部联接。
在 FROM 子句中指定外联接时，可以由下列几组关键字中的一组指定：
LEFT JOIN 或 LEFT OUTER JOIN。
左向外联接的结果集包括 LEFT OUTER 子句中指定的左表的所有行，而不仅仅是联接列所匹配的行。如果左表的某行在右表中没有匹配行，则在相关联的结果集行中右表的所有选择列表列均为空值。
RIGHT JOIN 或 RIGHT OUTER JOIN。
右向外联接是左向外联接的反向联接。将返回右表的所有行。如果右表的某行在左表中没有匹配行，则将为左表返回空值。
FULL JOIN 或 FULL OUTER JOIN。
完整外部联接返回左表和右表中的所有行。当某行在另一个表中没有匹配行时，则另一个表的选择列表列包含空值。如果表之间有匹配行，则整个结果集行包含基表的数据值。
交叉联接。
交叉联接返回左表中的所有行，左表中的每一行与右表中的所有行组合。交叉联接也称作笛卡尔积
MySql数据库中测试例子：
root
show databases;
use kali;
show tables;
--创建客户表
create table customer(
id int primary key auto_increment,
name varchar(10) not null,
address varchar(51) not null,
phone_number varchar(10) not null
);
insert into customer values(2,'kali','qingdao','123123123');
--创建消费表 (外键约束，fee表的customer_id为customer表的主键)
create table fee(
fee_id int primary key auto_increment,
customer_id int not null,
fee float(10),
foreign key(customer_id) references customer(id)
);
insert into fee values(2,1,'23');
customer表中的内容：
+----+------+------------+--------------+
| id | name | address | phone_number |
+----+------+------------+--------------+
| 1 | dxw | gongzhufen | 63983477 |
| 2 | kali | qingdao | 123123123 |
+----+------+------------+--------------+
fee表中的内容：
+--------+-------------+------+
| fee_id | customer_id | fee |
+--------+-------------+------+
| 1 | 1 | 1.5 |
| 2 | 1 | 23 |
+--------+-------------+------+
左连接：
select c.id,c.name,f.fee from customer as c left join fee as f on c.id=f.customer_id;
+----+------+------+
| id | name | fee |
+----+------+------+
| 1 | dxw | 1.5 |
| 1 | dxw | 23 |
| 2 | kali | NULL |
+----+------+------+
左外连接：
select c.id,c.name,f.fee from customer as c left outer join fee as f on c.id=f.customer_id;
+----+------+------+
| id | name | fee |
+----+------+------+
| 1 | dxw | 1.5 |
| 1 | dxw | 23 |
| 2 | kali | NULL |
+----+------+------+
右连接：
select c.id,c.name,f.fee from customer as c right join fee as f on c.id=f.customer_id;
+------+------+------+
| id | name | fee |
+------+------+------+
| 1 | dxw | 1.5 |
| 1 | dxw | 23 |
+------+------+------+
右外连接：
select c.id,c.name,f.fee from customer as c right outer join fee as f on c.id=f.customer_id;
+------+------+------+
| id | name | fee |
+------+------+------+
| 1 | dxw | 1.5 |
| 1 | dxw | 23 |
+------+------+------+
全连接：
select c.id,c.name,f.fee from customer as c,fee as f where c.id=f.customer_id;
select c.id,c.name,f,fee from customer as c full join fee as f on c.id=f.customer_id;(mysql中不支持全连接写法，这样在mysql中是不正确的)
+----+------+------+
| id | name | fee |
+----+------+------+
| 1 | dxw | 1.5 |
| 1 | dxw | 23 |
+----+------+------+
内连接：
--返回字段id与customer_id同时在customer表中也在fee表中。
select c.id,c.name,f.fee from customer as c inner join fee as f on c.id=f.customer_id;
+----+------+------+
| id | name | fee |
+----+------+------+
| 1 | dxw | 1.5 |
| 1 | dxw | 23 |
+----+------+------+
交叉连接：
--产生联接所涉及的表的笛卡尔积。第一个表的行数乘以第二个表的行数等于笛卡尔积结果集的大小
select c.id,c.name,f.fee from customer as c cross join fee as f;
select c.id,c.name,f.fee from customer as c,fee as f;
+----+------+------+
| id | name | fee |
+----+------+------+
| 1 | dxw | 1.5 |
| 2 | kali | 1.5 |
| 1 | dxw | 23 |
| 2 | kali | 23 |
+----+------+------+