内部联接两个表并从第三个表联接

Question

我有2个主要表格- posts和users ，还有第三个表格将用户喜欢的帖子存储为一行。 我需要对与用户数据结合在一起的帖子进行查询，还需要查询当前用户是否收藏了该帖子（在user_favs表中查找）。

最终，我想将以下内容返回给客户端：

{
    post_id: '...',
    title: '...',
    user_id: '...',
    username: '...',
    isFavourited: isFaved > 0 ? true : false
}

这可能与1选择查询吗？

CREATE TABLE posts (
    post_id    uuid,
    title      text
    author_id  uuid
);

CREATE TABLE users (
    user_id   uuid,
    username  text
);

CREATE TABLE user_favs (
    user_id     uuid REFERENCES users ON DELETE CASCADE,
    post_id  uuid REFERENCES posts ON DELETE CASCADE
);

我的查询看起来像：

SELECT
    p.post_id,
    p.title,
    u.user_id,
    u.username,

    count(f.*) as isfaved
FROM posts p
JOIN users u
ON p.author_id = u.user_id AND p.post_id = '1234'
LEFT JOIN user_favs f
ON f.user_id = p.author_id AND f.post_id = '1234'

而且我也尝试过：

SELECT
    p.post_id,
    p.title,
    u.user_id,
    u.username,
FROM posts p
JOIN users u
ON p.author_id = u.user_id AND p.post_id = '1234'
LEFT JOIN 
    (SELECT count(*) FROM user_favs
) f ON f.user_id = p.author_id AND f.post_id = '1234'

编辑：只是为了澄清我希望完成的工作：我正在获取一个帖子，返回该帖子的各自作者，然后还要检查该帖子是否受到请求用户的喜爱。

Answer 1

您最好将select count（*）放入SELECT子句中，类似于

SELECT
    p.post_id,
    p.title,
    u.user_id,
    u.username,
(SELECT count(*) FROM user_favs f where f.user_id = p.author_id AND f.post_id = p.post_id)
FROM posts p
JOIN users u
ON p.author_id = u.user_id AND p.post_id = '1234'

Answer 2

我改变了桌子，所以我可以测试它。 使用您提供的信息：为给定帖子选择帖子信息，作者以及当前用户（我猜为登录用户）是否偏爱该帖子：

drop table if exists user_favs cascade;
drop table if exists posts cascade;
drop table if exists users cascade; 

CREATE TABLE users (
    user_id   int primary key,
    username  text
);

CREATE TABLE posts (
    post_id    int primary key,
    title      text,
    user_id  int references users
);


CREATE TABLE user_favs (
    user_id     int REFERENCES users ON DELETE CASCADE,
    post_id  int REFERENCES posts ON DELETE CASCADE
);


SELECT
    p.post_id,
    p.title,
    u.user_id,
    u.username,

    case when f.user_id is not null then true else false end as isfaved
FROM posts p
JOIN users u ON p.user_id = u.user_id 
LEFT JOIN user_favs f on f.post_id = p.post_id
where p.post_id=1234 -- the post you want to select
and f.user_id = 4321; -- the id of the current user