多个选择连接的表与group by？

Question

我有三个表与结构概述如下：

CREATE TABLE users (
    id   BIGSERIAL PRIMARY KEY, 
    username VARCHAR(255) UNIQUE 
); 
CREATE TABLE posts (
    id   BIGSERIAL PRIMARY KEY, 
    user_id  BIGINT REFERENCES users(id) NOT NULL, 
    category BIGINT REFERENCES categories(id) NOT NULL, 
    text  TEXT NOT NULL 
); 
CREATE TABLE posts_votes (
    user_id  BIGINT REFERENCES users(id) NOT NULL, 
    post_id  BIGINT REFERENCES posts(id) NOT NULL 
    value  SMALLINT NOT NULL, 
    PRIMARY KEY(user_id, post_id) 
); 

我能够编写一个查询，得到每个岗位，其用户和使用下面的查询其总价值：

SELECT p.id, p.text, u.username, COALESCE(SUM(v.value), 0) AS vote_value 
FROM posts p 
    LEFT JOIN posts_votes v ON p.id=t.post_id 
    JOIN users u ON p.user_id=u.id 
WHERE posts.category=1337 
GROUP BY p.id, p.text, u.username 

但现在我想返回一个列，返回上述查询中每个post_id n的结果SELECT COALESCE((SELECT value FROM posts_votes WHERE user_id=1234 AND post_id=n), 0)。什么是最好的方法来做到这一点？

Answer 1

我认为一个额外的LEFT JOIN是一种合理的方法：

SELECT p.id, p.text, u.username, COALESCE(SUM(v.value), 0) AS vote_value, 
     COALESCE(pv.value, 0) 
FROM posts p JOIN 
    users u 
    ON p.user_id=u.id LEFT JOIN 
    topics_votes v 
    ON p.id = t.post_id LEFT JOIN 
    post_votes pv 
    ON pv.user_id = 1234 AND pv.post_id = p.id 
WHERE p.category = 1337 
GROUP BY p.id, p.text, u.username, pv.value;