使用 PostgreSQL/NodeJS 获取 JOIN 表作为结果数组

本文介绍了使用 PostgreSQL/NodeJS 获取 JOIN 表作为结果数组的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 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，其他人可以对它们进行投票/否决.

I'm creating an app where users are able to create questions, and others can upvote/downvote them.

以下是我的sql架构的一部分:

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 );

我想要的是 Postgres 给我一个投票数组，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(如 array_agg())，但它只给了我值.A JOIN 给了我一个加入投票的问题，并会迫使我进行服务器端操作，而我不想这样做.

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?我对我想要获得的东西的推理是错误的吗?

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 承诺:

function buildTree(t) { const v = q => t.any('SELECT id, value FROM votes WHERE question_id = $1', q.id) .then(votes => { q.votes = votes; return q; }); return t.map('SELECT * FROM questions', undefined, v).then(a => t.batch(a)); } db.task(buildTree) .then(data => { console.log(data); // your data tree }) .catch(error => { console.log(error); });

同上，但使用 ES7 async/await 语法:

The same as above, but using ES7 async/await syntax:

await db.task(async t => { const questions = await t.any('SELECT * FROM questions'); for(const q of questions) { q.votes = await t.any('SELECT id, value FROM votes WHERE question_id = $1', [q.id]); } return questions; }); // method "task" resolves with the correct data tree

API:地图、any, task, 批处理

相关问题:

• 使用 pg-promise 获取父级 + 子级树
• 使用 pg-promise 的条件任务

如果您只想使用单个查询，那么使用 PostgreSQL 9.4 及更高版本的语法，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

然后你的 pg-promise 示例是:

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`; const data = await db.map(query, [], a => a.json);

而且您肯定希望将如此复杂的查询保留在外部 SQL 文件中.请参阅查询文件.

And you definitely will want to keep such complex queries in external SQL files. See Query Files.

在上述两种方法之间进行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:

• 单查询方法更快，但有点难以阅读或扩展，相当冗长
• 多查询方法更易于理解和扩展，但由于执行的查询数量是动态的，因此性能不佳.

UPDATE-1

以下相关答案通过连接子查询提供了更多选项，这将大大提高性能:将嵌套循环查询组合到父结果 pg-promise.

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.

UPDATE-2

添加了另一个示例，使用 ES7 async/await 方法.

Another example added, using ES7 async/await approach.