如何在 SQL 中将这 3 个表连接在一起？

Question

所以我总共有 3 个表，分别称为 Companies_company c、company_sharedtrainingunit cst 和 Companies_trainingunit ct。我创建的第一个 JOIN 在同一个公司 ID 上加入了 c 和 cst。

SELECT c.id as consuming_company_id, c.name as consuming_company_name, cst.id as channel_connection_id, cst.training_unit_id as channel_id
FROM companies_sharedtrainingunit cst
FULL JOIN companies_company c ON c.id = cst.company_id
ORDER BY company_id ASC

生成一个表格，结果如下

consuming_company_id	consuming_company_name	channel_connection_id	channel_id
1	company_1	2000	100

现在我希望上面创建的这个连接表连接到来自表 c 的相同信息和一个新表 ct ie

SELECT c.id as owned_company_id, c.name as owned_company_name, ct.id as channel_id, ct.name as channel_name, ct.description as channel_description
FROM companies_trainingunit ct
FULL JOIN companies_company c ON c.id = ct.company_id
ORDER BY c.id

owned_company_id	owned_company_name	channel_id	channel_name	channel_description
2	company_2	100	channel_100	description_100

我已经尝试加入这两个查询但没有运气:(

我知道他们需要加入他们的频道 ID

最后我正在寻找我的桌子看起来像：

channel_connection_id	consuming_company_id	consuming_company_name	channel_id	channel_name	channel_description	owned_company_id	owned_company_name
2000	1	company_1	100	channel_100	description_100	2	company_2

Answer 1

看来您需要加入 companies_company 两次。此外，内部连接就足够了：

SELECT cst.id as channel_connection_id,
       c1.id as consuming_company_id,
       c1.name as consuming_company_name,
       cst.training_unit_id as channel_id,
       ct.name as channel_name,
       ct.description as channel_description,
       c2.id as owned_company_id,
       c2.name as owned_company_name,
FROM companies_sharedtrainingunit cst
   JOIN companies_company c1 ON c1.id = cst.company_id
   JOIN companies_trainingunit ct ON cst.training_unit_id = ct.id
   JOIN companies_company c2 ON c2.id = ct.company_id
ORDER BY cst.company_id ASC;

Answer 2

我发现很难理解您的数据模型。有名称中带有“trainingunit”的表，但没有 trainingunit ID。相反，您要加入一个“channel_id”。这是为什么呢？

然后，您将使用完全外部联接来包括没有培训单位的公司和没有公司的培训单位。这有意义吗？

但是，如果您的查询是正确的并且您想加入它们，那么就这样做吧：

select *
from
(
  SELECT c.id as consuming_company_id, c.name as consuming_company_name, cst.id as channel_connection_id, cst.training_unit_id as channel_id
  FROM companies_sharedtrainingunit cst
  FULL JOIN companies_company c ON c.id = cst.company_id
) q1
(
  SELECT c.id as owned_company_id, c.name as owned_company_name, ct.id as channel_id, ct.name as channel_name, ct.description as channel_description
  FROM companies_trainingunit ct
  FULL JOIN companies_company c ON c.id = ct.company_id
) q2 using (channel_id)
order by channel_id;

Answer 3

我希望以companies 开头的left join 足以获得您想要的行。这符合您的要求吗？

SELECT c.id as consuming_company_id, c.name as consuming_company_name,
       cst.id as channel_connection_id, cst.training_unit_id as channel_id,
       ct.id as channel_id, ct.name as channel_name, ct.description as channel_description
FROM companies_company c LEFT JOIN
     companies_sharedtrainingunit cst
     ON c.id = cst.company_id LEFT JOIN
     companies_trainingunit ct
     ON c.id = ct.company_id     
ORDER BY c.company_id ASC;