将一个表连接到两个单独的一对多表

Question

我有三个表，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        |

我想写的是一个查询，它给我恰好十个结果：基本上是widgets和sprockets每一行一个，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，我的联接正在组合以返回12行。 添加一个by似乎减少了太多。

我已经尝试过类似以下的方法，但是联接却成倍增加：

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;

我 发现的答案主要集中在重新设计数据库上，但这对我来说不是一个选择。 如何编写联接以从该数据集中返回以上十行？

Answer 1

对每个表使用联接的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