上次面试被问到join，自己都已经忘了课堂上讲的笛卡尔积那些就是join，最近重新复习了一遍
join的含义就如英文单词“join”一样，连接两张表，大致分为内连接，外连接，右连接，左连接，自然连接。
先创建两个表，下面用于示例
create table t_blog(
id int primary key auto_increment,
title varchar(50),
typeid int
);
select * from t_blog;
+----+-------+--------+
| id | title | typeid |
+----+-------+--------+
| 1 | aaa | 1 |
| 2 | bbb | 2 |
| 3 | ccc | 3 |
| 4 | ddd | 4 |
| 5 | eee | 4 |
| 6 | fff | 3 |
| 7 | ggg | 2 |
| 8 | hhh | null |
| 9 | iii | null |
| 10 | jjj | null |
+----+-------+--------+
-- 博客的类别
create table t_type(
id int primary key auto_increment,
name varchar(20)
);
select * from t_type;
+----+------------+
| id | name |
+----+------------+
| 1 | c++ |
| 2 | c |
| 3 | java |
| 4 | c# |
| 5 | javascript |
+----+------------+
笛卡尔积：cross join
要理解各种join首先要理解笛卡尔积。笛卡尔积就是将a表的每一条记录与b表的每一条记录强行拼在一起。所以，如果a表有n条记录，b表有m条记录，笛卡尔积产生的结果就会产生n*m条记录。下面的例子，t_blog有10条记录，t_type有5条记录，所有他们俩的笛卡尔积有50条记录。有五种产生笛卡尔积的方式如下。
select * from t_blog cross join t_type;
select * from t_blog inner join t_type;
select * from t_blog,t_type;
select * from t_blog nature join t_type;
select * from t_blog natura join t_type;
+----+-------+--------+----+------------+
| id | title | typeid | id | name |
+----+-------+--------+----+------------+
| 1 | aaa | 1 | 1 | c++ |
| 1 | aaa | 1 | 2 | c |
| 1 | aaa | 1 | 3 | java |
| 1 | aaa | 1 | 4 | c# |
| 1 | aaa | 1 | 5 | javascript |
| 2 | bbb | 2 | 1 | c++ |
| 2 | bbb | 2 | 2 | c |
| 2 | bbb | 2 | 3 | java |
| 2 | bbb | 2 | 4 | c# |
| 2 | bbb | 2 | 5 | javascript |
| 3 | ccc | 3 | 1 | c++ |
| 3 | ccc | 3 | 2 | c |
| 3 | ccc | 3 | 3 | java |
| 3 | ccc | 3 | 4 | c# |
| 3 | ccc | 3 | 5 | javascript |
| 4 | ddd | 4 | 1 | c++ |
| 4 | ddd | 4 | 2 | c |
| 4 | ddd | 4 | 3 | java |
| 4 | ddd | 4 | 4 | c# |
| 4 | ddd | 4 | 5 | javascript |
| 5 | eee | 4 | 1 | c++ |
| 5 | eee | 4 | 2 | c |
| 5 | eee | 4 | 3 | java |
| 5 | eee | 4 | 4 | c# |
| 5 | eee | 4 | 5 | javascript |
| 6 | fff | 3 | 1 | c++ |
| 6 | fff | 3 | 2 | c |
| 6 | fff | 3 | 3 | java |
| 6 | fff | 3 | 4 | c# |
| 6 | fff | 3 | 5 | javascript |
| 7 | ggg | 2 | 1 | c++ |
| 7 | ggg | 2 | 2 | c |
| 7 | ggg | 2 | 3 | java |
| 7 | ggg | 2 | 4 | c# |
| 7 | ggg | 2 | 5 | javascript |
| 8 | hhh | null | 1 | c++ |
| 8 | hhh | null | 2 | c |
| 8 | hhh | null | 3 | java |
| 8 | hhh | null | 4 | c# |
| 8 | hhh | null | 5 | javascript |
| 9 | iii | null | 1 | c++ |
| 9 | iii | null | 2 | c |
| 9 | iii | null | 3 | java |
| 9 | iii | null | 4 | c# |
| 9 | iii | null | 5 | javascript |
| 10 | jjj | null | 1 | c++ |
| 10 | jjj | null | 2 | c |
| 10 | jjj | null | 3 | java |
| 10 | jjj | null | 4 | c# |
| 10 | jjj | null | 5 | javascript |
+----+-------+--------+----+------------+
内连接：inner join
内连接inner join是最常用的连接操作。从数学的角度讲就是求两个表的交集，从笛卡尔积的角度讲就是从笛卡尔积中挑出on子句条件成立的记录。有inner join，where（等值连接），straight_join,join(省略inner)四种写法。
select * from t_blog inner join t_type on t_blog.typeid=t_type.id;
select * from t_blog,t_type where t_blog.typeid=t_type.id;
select * from t_blog straight_join t_type on t_blog.typeid=t_type.id; --注意stright_join有个下划线
select * from t_blog join t_type on t_blog.typeid=t_type.id;
+----+-------+--------+----+------+
| id | title | typeid | id | name |
+----+-------+--------+----+------+
| 1 | aaa | 1 | 1 | c++ |
| 2 | bbb | 2 | 2 | c |
| 7 | ggg | 2 | 2 | c |
| 3 | ccc | 3 | 3 | java |
| 6 | fff | 3 | 3 | java |
| 4 | ddd | 4 | 4 | c# |
| 5 | eee | 4 | 4 | c# |
+----+-------+--------+----+------+
左连接：left join
左连接left join的含义就是求两个表的交集外加左表剩下的数据。依旧从笛卡尔积的角度讲，就是先从笛卡尔积中挑出on子句条件成立的记录，然后加上左表中剩余的记录（见最后三条）。
select * from t_blog left join t_type on t_blog.typeid=t_type.id;
+----+-------+--------+------+------+
| id | title | typeid | id | name |
+----+-------+--------+------+------+
| 1 | aaa | 1 | 1 | c++ |
| 2 | bbb | 2 | 2 | c |
| 7 | ggg | 2 | 2 | c |
| 3 | ccc | 3 | 3 | java |
| 6 | fff | 3 | 3 | java |
| 4 | ddd | 4 | 4 | c# |
| 5 | eee | 4 | 4 | c# |
| 8 | hhh | null | null | null |
| 9 | iii | null | null | null |
| 10 | jjj | null | null | null |
+----+-------+--------+------+------+
右连接：right join
同理右连接right join就是求两个表的交集外加右表剩下的数据。再次从笛卡尔积的角度描述，右连接就是从笛卡尔积中挑出on子句条件成立的记录，然后加上右表中剩余的记录（见最后一条）。
select * from t_blog right join t_type on t_blog.typeid=t_type.id;
+------+-------+--------+----+------------+
| id | title | typeid | id | name |
+------+-------+--------+----+------------+
| 1 | aaa | 1 | 1 | c++ |
| 2 | bbb | 2 | 2 | c |
| 3 | ccc | 3 | 3 | java |
| 4 | ddd | 4 | 4 | c# |
| 5 | eee | 4 | 4 | c# |
| 6 | fff | 3 | 3 | java |
| 7 | ggg | 2 | 2 | c |
| null | null | null | 5 | javascript |
+------+-------+--------+----+------------+
外连接：outer join
外连接就是求两个集合的并集。从笛卡尔积的角度讲就是从笛卡尔积中挑出on子句条件成立的记录，然后加上左表中剩余的记录，最后加上右表中剩余的记录。另外mysql不支持outer join，但是我们可以对左连接和右连接的结果做union操作来实现。
select * from t_blog left join t_type on t_blog.typeid=t_type.id
union
select * from t_blog right join t_type on t_blog.typeid=t_type.id;
+------+-------+--------+------+------------+
| id | title | typeid | id | name |
+------+-------+--------+------+------------+
| 1 | aaa | 1 | 1 | c++ |
| 2 | bbb | 2 | 2 | c |
| 7 | ggg | 2 | 2 | c |
| 3 | ccc | 3 | 3 | java |
| 6 | fff | 3 | 3 | java |
| 4 | ddd | 4 | 4 | c# |
| 5 | eee | 4 | 4 | c# |
| 8 | hhh | null | null | null |
| 9 | iii | null | null | null |
| 10 | jjj | null | null | null |
| null | null | null | 5 | javascript |
+------+-------+--------+------+------------+
using子句
mysql中连接sql语句中，on子句的语法格式为：table1.column_name = table2.column_name。当模式设计对联接表的列采用了相同的命名样式时，就可以使用 using 语法来简化 on 语法，格式为：using(column_name)。
所以，using的功能相当于on，区别在于using指定一个属性名用于连接两个表，而on指定一个条件。另外，select *时，using会去除using指定的列，而on不会。实例如下。
select * from t_blog inner join t_type on t_blog.typeid =t_type.id;
+----+-------+--------+----+------+
| id | title | typeid | id | name |
+----+-------+--------+----+------+
| 1 | aaa | 1 | 1 | c++ |
| 2 | bbb | 2 | 2 | c |
| 7 | ggg | 2 | 2 | c |
| 3 | ccc | 3 | 3 | java |
| 6 | fff | 3 | 3 | java |
| 4 | ddd | 4 | 4 | c# |
| 5 | eee | 4 | 4 | c# |
+----+-------+--------+----+------+
select * from t_blog inner join t_type using(typeid);
error 1054 (42s22): unknown column 'typeid' in 'from clause'
select * from t_blog inner join t_type using(id); -- 应为t_blog的typeid与t_type的id不同名，无法用using，这里用id代替下。
+----+-------+--------+------------+
| id | title | typeid | name |
+----+-------+--------+------------+
| 1 | aaa | 1 | c++ |
| 2 | bbb | 2 | c |
| 3 | ccc | 3 | java |
| 4 | ddd | 4 | c# |
| 5 | eee | 4 | javascript |
+----+-------+--------+------------+
自然连接：nature join
自然连接就是using子句的简化版，它找出两个表中相同的列作为连接条件进行连接。有左自然连接，右自然连接和普通自然连接之分。在t_blog和t_type示例中，两个表相同的列是id，所以会拿id作为连接条件。
另外千万分清下面三条语句的区别 。
自然连接:select * from t_blog natural join t_type;
笛卡尔积:select * from t_blog natura join t_type;
笛卡尔积:select * from t_blog nature join t_type;
select * from t_blog natural join t_type;
select t_blog.id,title,typeid,t_type.name from t_blog,t_type where t_blog.id=t_type.id;
select t_blog.id,title,typeid,t_type.name from t_blog inner join t_type on t_blog.id=t_type.id;
select t_blog.id,title,typeid,t_type.name from t_blog inner join t_type using(id);
+----+-------+--------+------------+
| id | title | typeid | name |
| 1 | aaa | 1 | c++ |
| 2 | bbb | 2 | c |
| 3 | ccc | 3 | java |
| 4 | ddd | 4 | c# |
| 5 | eee | 4 | javascript |
select * from t_blog natural left join t_type;
select t_blog.id,title,typeid,t_type.name from t_blog left join t_type on t_blog.id=t_type.id;
select t_blog.id,title,typeid,t_type.name from t_blog left join t_type using(id);
| 6 | fff | 3 | null |
| 7 | ggg | 2 | null |
| 8 | hhh | null | null |
| 9 | iii | null | null |
| 10 | jjj | null | null |
select * from t_blog natural right join t_type;
select t_blog.id,title,typeid,t_type.name from t_blog right join t_type on t_blog.id=t_type.id;
select t_blog.id,title,typeid,t_type.name from t_blog right join t_type using(id);
+----+------------+-------+--------+
| id | name | title | typeid |
| 1 | c++ | aaa | 1 |
| 2 | c | bbb | 2 |
| 3 | java | ccc | 3 |
| 4 | c# | ddd | 4 |
| 5 | javascript | eee | 4 |
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