mysql left join right join inner join例子

建表语句如下：

    #--------------create table_a-----------
create table table_a(
id int(11) not null auto_increment,
name varchar(20) not null,
primary key(id)
)engine=InnoDB default charset=utf8;

#----------------create table_b----------
create table table_b(
id int(11) not null,
name varchar(20) not null,
primary key(id)
)engine=InnoDB default charset=utf8;

插入几条数据：
#-------------insert data into table a------------
insert into table_a values(1,'a20121313'),(2,'a2131314'),(3,'a43239284392'),(4,'a33727427'),
(5,'a32382592'),(6,'a42424224'),(7,'a14312141');
#-------insert data into table_b----------------
insert into table_b values(1,'b2141414'),(2,'b15315151'),(3,'b562842842'),(4,'b32185115');
1.left join

sql如下：

 select * from table_a a left join table_b b on a.id = b.id;

结果如下：
 +----+--------------+------+------------+
| id | name | id | name |
+----+--------------+------+------------+
| 1 | a20121313 | 1 | b2141414 |
| 2 | a2131314 | 2 | b15315151 |
| 3 | a43239284392 | 3 | b562842842 |
| 4 | a33727427 | 4 | b32185115 |
| 5 | a32382592 | NULL | NULL |
| 6 | a382331 | NULL | NULL |
| 7 | a64376371631 | NULL | NULL |
+----+--------------+------+------------+
7 rows in set (0.00 sec)
通过观察结果可以得出以下结论，left join以左表为基础表，以右表为从表，左表不存在的数据设置为NULL。
2.right join

sql如下：

select * from table_a a right join table_b b on a.id = b.id;


结果如下:
+------+--------------+----+------------+
| id | name | id | name |
+------+--------------+----+------------+
| 1 | a20121313 | 1 | b2141414 |
| 2 | a2131314 | 2 | b15315151 |
| 3 | a43239284392 | 3 | b562842842 |
| 4 | a33727427 | 4 | b32185115 |
+------+--------------+----+------------+
4 rows in set (0.00 sec)


right join以右表为基础表，左表为从表，左表不存在的项设置为NULL
3.inner join

inner join无基础表，只是显示符合条件的结果。