MySQL 7种 JOIN连表方法

规定：左边的圆代表表 a，右边的代表 b。

JOIN 关键字可以在两表之间选中任意部分。】

通过以下代码制造一些数据：

delimiter //
drop procedure if exists produce_data//
create procedure produce_data()
begin
declare i int default 0;
drop table if exists a;
drop table if exists b;
create table a(id int not null,name varchar(32));
create table b(id int not null,name varchar(32));
set i = 1;
while i <= 4 do
insert into a(id,name) values(i, concat('name', i));
set i = i + 1;
end while;
set i = 3;
while i <= 6 do
insert into b(id,name) values(i, concat('name', i));
set i = i + 1;
end while;
end//
call produce_data()//

mysql> select * from a//
+----+-------+
| id | name |
+----+-------+
| 1 | name1 |
| 2 | name2 |
| 3 | name3 |
| 4 | name4 |
+----+-------+
mysql> select * from b//
+----+-------+
| id | name |
+----+-------+
| 3 | name3 |
| 4 | name4 |
| 5 | name5 |
| 6 | name6 |
+----+-------+

select *
from a left join b on a.id = b.id//


+----+-------+------+-------+
| id | name | id | name |
+----+-------+------+-------+
| 1 | name1 | NULL | NULL |
| 2 | name2 | NULL | NULL |
| 3 | name3 | 3 | name3 |
| 4 | name4 | 4 | name4 |
+----+-------+------+-------+

select *
from a right outer join b on a.id = b.id//
 

+------+-------+----+-------+
| id | name | id | name |
+------+-------+----+-------+
| 3 | name3 | 3 | name3 |
| 4 | name4 | 4 | name4 |
| NULL | NULL | 5 | name5 |
| NULL | NULL | 6 | name6 |
+------+-------+----+-------+

select *
from a inner join b on a.id = b.id//

+----+-------+----+-------+
| id | name | id | name |
+----+-------+----+-------+
| 3 | name3 | 3 | name3 |
| 4 | name4 | 4 | name4 |
+----+-------+----+-------+

select *
from a left join b on a.id = b.id
where b.id is null//

+----+-------+------+------+
| id | name | id | name |
+----+-------+------+------+
| 1 | name1 | NULL | NULL |
| 2 | name2 | NULL | NULL |
+----+-------+------+------+

select *
from a right join b on a.id = b.id
where a.id is null//

+------+------+----+-------+
| id | name | id | name |
+------+------+----+-------+
| NULL | NULL | 5 | name5 |
| NULL | NULL | 6 | name6 |
+------+------+----+-------+

一般，是这样写：

select *
from a full outer join b on a.id = b.id
where a.id is null or b.id is null//

但是，mysql 并没有 FULL 关键字，因此使用 UNION 联接 左连接和 右连接。

select *
from a left join b on a.id = b.id
where b.id is null
union
select *
from a right join b on a.id = b.id
where a.id is null//
+------+--------+------+--------+
| a_id | a_name | b_id | b_name |
+------+--------+------+--------+
| 1 | name1 | NULL | NULL |
| 2 | name2 | NULL | NULL |
| NULL | NULL | 5 | name5 |
| NULL | NULL | 6 | name6 |
+------+--------+------+--------+

类似上面，使用UNION

select a.id a_id, a.name a_name, b.id b_id, b.name b_name
from a left join b on a.id = b.id
union
select a.id a_id, a.name a_name, b.id b_id, b.name b_name
from a right join b on a.id = b.id//

+------+--------+------+--------+
| a_id | a_name | b_id | b_name |
+------+--------+------+--------+
| 1 | name1 | NULL | NULL |
| 2 | name2 | NULL | NULL |
| 3 | name3 | 3 | name3 |
| 4 | name4 | 4 | name4 |
| NULL | NULL | 5 | name5 |
| NULL | NULL | 6 | name6 |
+------+--------+------+--------+