使用PostgreSQL/NodeJS获取JOIN表作为结果数组 [英] get JOIN table as array of results with PostgreSQL/NodeJS
问题描述
我正在创建一个应用程序，用户可以在其中创建问题，其他人可以对其进行投票/降级投票.
I'm creating an app where users are able to create questions, and others can upvote/downvote them.
以下是我的sql模式的一部分:
The following is a part of my sql schema:
CREATE TABLE "questions" (
id SERIAL,
content VARCHAR(511) NOT NULL,
created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW(),
CONSTRAINT pk_question PRIMARY KEY (id)
);
CREATE TABLE "votes" (
id SERIAL,
value INT,
question_id INT NOT NULL,
CONSTRAINT pk_vote PRIMARY KEY (id),
CONSTRAINT fk_question_votes FOREIGN KEY (question_id) REFERENCES questions (id) MATCH SIMPLE ON UPDATE CASCADE ON DELETE CASCADE
);
我想拥有的是Postgres给我每个问题一个投票，就像这样:
What I would like to have is Postgres giving me each question with an array of votes, like that:
[{ // a question
id: 1,
content: 'huh?',
votes: [{ // a vote
id: 1,
value: 1
}, { // another vote
id: 2,
value: -1
}]
}, { /*another question with votes*/ }]
我查看了聚合函数(例如array_agg())，但它仅提供了值. JOIN给我一个问题，并投了赞成票，这将迫使我进行服务器端操作，而我不希望这样做.
I looked at aggregate functions (like array_agg()) but it gave me only the values. A JOIN gave me a question joined with a vote, and would force me to do server side operations, which I would prefer not to.
有没有办法做到这一点?我对想要得到的东西有错误的看法吗?
Is there any way to do that? Is my reasoning regarding what I want to obtain wrong?
感谢您的时间.
推荐答案
使用 pg-promise :
function buildTree(t) {
return t.map('SELECT * FROM questions', [], q => {
return t.any('SELECT id, value FROM votes WHERE question_id = $1', q.id)
.then(votes => {
q.votes = votes;
return q;
});
}).then(t.batch); // settles the array of generated promises
}
db.task(buildTree)
.then(data => {
console.log(data); // your data tree
})
.catch(error => {
console.log(error);
});
- Get a parents + children tree with pg-promise
- Conditional task with pg-promise
如果只想使用一个查询，那么使用PostgreSQL 9.4和更高版本的语法，您可以执行以下操作:
And if you want to use just a single query, then using PostgreSQL 9.4 and later syntax you can do the following:
SELECT json_build_object('id', q.id, 'content', q.content, 'votes',
(SELECT json_agg(json_build_object('id', v.id, 'value', v.value))
FROM votes v WHERE q.id = v.question_id))
FROM questions q
然后您的 pg-promise 示例将是:
const query =
`SELECT json_build_object('id', q.id, 'content', q.content, 'votes',
(SELECT json_agg(json_build_object('id', v.id, 'value', v.value))
FROM votes v WHERE q.id = v.question_id)) json
FROM questions q`;
db.map(query, [], a => a.json)
.then(data => {
console.log(data); // your data tree
})
.catch(error => {
console.log(error);
});
您肯定会希望将此类复杂的查询保留在外部SQL文件中.参见查询文件.
And you definitely will want to keep such complex queries in external SQL files. See Query Files.
以上两种方法之间的选择应基于您的应用程序的性能要求:
The choice between the two approaches presented above should be based on the performance requirements of your application:
- 单查询方法速度更快，但是相当冗长，但难以读取或扩展
- 多查询方法更易于理解和扩展，但由于执行的查询数量动态，因此对性能的影响不大.
更新
以下相关答案通过串联子查询提供了更多选项，这将大大提高性能:
The following related answer offers more options, by concatenating child queries, which will give a much improved performance: Combine nested loop queries to parent result pg-promise.
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