Question

假设我在PostgreSQL中有这两个表。

TABLE "Likes" (
    id,
    post-id (foreign key pointing to "Post" table)
)

TABLE "Post" (
    id,
    body,
    date
)

我想要做的是获取所有帖子和每个帖子的喜欢数量。

我写了这个查询：

SELECT "post-id", COUNT(*) AS "likes" FROM "Likes"
GROUP BY "post-id";

将返回以下内容：

-------------------
| post-id | likes |
-------------------
| 1       | 9     |
-------------------
| 4       | 2     |
-------------------

但我仍然需要将此查询与获取有关帖子的所有信息的查询合并...所以最后，我想要这样的事情：

---------------------------------------------------
| id | likes | body                  | date       |
---------------------------------------------------
| 1  | 9     | This is a post!       | 2017-05-03 |
---------------------------------------------------
| 2  |       | Tasdasdas!            | 2017-05-03 |
---------------------------------------------------
| 4  | 2     | This is another post! | 2017-05-04 |
---------------------------------------------------
| 5  |       | ssdasdasdadadass      | 2017-05-04 |
---------------------------------------------------

我的问题是，我不知道如何添加＆＃34;喜欢＆＃34;获取所有帖子的查询列，因为只有某些帖子喜欢。

提前致谢！ ^ _ ^

Answer 1

您想要的是LEFT JOIN的典型案例：

SELECT    post.*, COUNT(likes.id) likes
FROM      post
LEFT JOIN likes ON post.id = likes.post_id
GROUP BY  post.id

这样，您可以SELECT post.*，因为GROUP BY post.id意味着functional dependency。

Sidenote ：您的标识符名称非常奇怪。您应该尽量避免引用标识符，这只会让您的查询更难阅读。

Answer 2

Select p.id, c."likes", p.body, p.date
from Posts p
left join
(
SELECT "post-id", COUNT(*) AS "likes" FROM "Likes"
GROUP BY "post-id"
) c on p.id = c."post-id";

如何合并两个不同的查询

2 个答案: