mysql left join join right

create table java (name varchar(255));
insert into java values ('java1'),('java2'),('blue'); 
create table mysql (name varchar(255));
insert into mysql values ('mysql1'),('mysql2'),('blue'); 

1.join 查询 内联查询 查两个表的内才联系相同的值

select * from java join  mysql as my on my.name = java.name; 

+------+------+
| name | name |
+------+------+
| blue | blue |
+------+------+
1 row in set (0.00 sec)查询结果 

2.left join 外联查询  已左表为主表 左表的信息全部会全部查出来  右表的信息只有相关联的才能查出来  剩下的查询结果为null 效率慢一点

select * from java left join mysql as my on java.name=my.name;


+-------+------+
| name  | name |
+-------+------+
| java1 | NULL |
| java2 | NULL |
| blue  | blue |
+-------+------+
3 rows in set (0.00 sec)  查询结果

3.rignt  join 正好与left join 相反