加入一个表，两个独立的一个一对多表

Question

我有三个表，就像这样：

projects 
| id | name  | 
|----|------------| 
| 1 | enterprise | 
| 2 | discovery | 

widgets 
| project_id | name   | 
|------------|-----------------| 
| 1   | saucer section | 
| 1   | pylons   | 
| 2   | spinning saucer | 
| 2   | angular pylons | 

sprockets 
| project_id | name  | 
|------------|-------------| 
| 1   | engineering | 
| 1   | bridge  | 
| 1   | mess  | 
| 2   | engineering | 
| 2   | bridge  | 
| 2   | mess  | 

我想写的是，给我正好十个结果的查询：基本上是一个为每行widgets和sprockets，看起来像这样：

result 
| project_name | widget_name  | sprocket_name | 
|--------------|-----------------|---------------| 
| enterprise | saucer section | null   | 
| enterprise | pylons   | null   | 
| enterprise | null   | engineering | 
| enterprise | null   | bridge  | 
| enterprise | null   | mess   | 
| discovery | spinning saucer | null   | 
| discovery | angular pylons | null   | 
| discovery | null   | engineering | 
| discovery | null   | bridge  | 
| discovery | null   | mess   | 

相反，我的连接被合并到返回12行。增加一个组似乎减少了太多。

我已经试过类似以下，但连接被乘以：

select 
    p.name as project_name, 
    w.name as widget_name, 
    s.name as sprocket_name 
from 
    projects as p 
left join widgets w on p.id = w.project_id 
left join sprockets s on p.id = s.project_id; 

的answersI'vefound主要集中在重新设计的数据库，但是这不是我的选择。 如何编写连接以从此数据集返回上述十行？

Answer 1

使用与每个表连接的UNION。

SELECT p.name AS project_name, w.name AS widget_name, NULL AS sprocket_name 
FROM projects AS p 
INNER JOIN widgets AS w ON p.id = w.project_id 

UNION ALL 

SELECT p.name AS project_name, NULL AS widget_name, s.name AS sprocket_name 
FROM projects AS p 
INNER JOIN sprockets AS s ON p.id = s.project_id