简单介绍
left join(左联接)： 返回包括左表中的所有记录和右表中联结字段相等的记录
right join(右联接) ：返回包括右表中的所有记录和左表中联结字段相等的记录
inner join(等值连接) ：只返回两个表中联结字段相等的行
用法
假设数据库中有2个表，myclass和Class，分别有如下内容：
MariaDB [form]> select * from myclass;
+----+------------+----------+
| id | phone | name |
+----+------------+----------+
| 1 | 2147483647 | xiaoming |
| 2 | 2147483647 | xiaowu |
| 3 | 2147483647 | xiaotian |
+----+------------+----------+
3 rows in set (0.00 sec)
MariaDB [form]> select * from Class;
+----+----------+-------+-------+-----+
| id | name | sex | score | age |
+----+----------+-------+-------+-----+
| 1 | xiaoming | man | 80 | 0 |
| 2 | xiaowu | man | 65 | 0 |
| 3 | xiaotian | woman | 77 | 18 |
| 4 | xiaohu | 0 | 88 | 21 |
+----+----------+-------+-------+-----+
4 rows in set (0.00 sec)
left join 的用法：
MariaDB [form]> select * from Class left join myclass on Class.id=myclass.id;
+----+----------+-------+-------+-----+------+------------+----------+
| id | name | sex | score | age | id | phone | name |
+----+----------+-------+-------+-----+------+------------+----------+
| 1 | xiaoming | man | 80 | 0 | 1 | 2147483647 | xiaoming |
| 2 | xiaowu | man | 65 | 0 | 2 | 2147483647 | xiaowu |
| 3 | xiaotian | woman | 77 | 18 | 3 | 2147483647 | xiaotian |
| 4 | xiaohu | 0 | 88 | 21 | NULL | NULL | NULL |
+----+----------+-------+-------+-----+------+------------+----------+
right join 的用法：
MariaDB [form]> select * from Class right join myclass on Class.id=myclass.id;
+------+----------+-------+-------+------+----+------------+----------+
| id | name | sex | score | age | id | phone | name |
+------+----------+-------+-------+------+----+------------+----------+
| 1 | xiaoming | man | 80 | 0 | 1 | 2147483647 | xiaoming |
| 2 | xiaowu | man | 65 | 0 | 2 | 2147483647 | xiaowu |
| 3 | xiaotian | woman | 77 | 18 | 3 | 2147483647 | xiaotian |
+------+----------+-------+-------+------+----+------------+----------+
inner join 的用法：
MariaDB [form]> select * from Class inner join myclass on Class.id=myclass.id;
+----+----------+-------+-------+-----+----+------------+----------+
| id | name | sex | score | age | id | phone | name |
+----+----------+-------+-------+-----+----+------------+----------+
| 1 | xiaoming | man | 80 | 0 | 1 | 2147483647 | xiaoming |
| 2 | xiaowu | man | 65 | 0 | 2 | 2147483647 | xiaowu |
| 3 | xiaotian | woman | 77 | 18 | 3 | 2147483647 | xiaotian |
+----+----------+-------+-------+-----+----+------------+----------+