如何连接多个外键？

Question

我正在使用PHP和SQL创建一个博客，我试图在1个SQL查询中查询所有内容，以便我可以带出作者的所有评论以及带作者的所有博客。他们每个人都有user.uuid作为外键。

我的博客表如下：

CREATE TABLE `blogs` (
  `uuid` int(255) NOT NULL,
  `title` varchar(1024) NOT NULL,
  `detail` varchar(1024) NOT NULL,
  `user_id` int(255) NOT NULL,
  `created` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

我的博客评论表如下：

CREATE TABLE `blogs_comments` (
  `uuid` int(255) NOT NULL,
  `blog_uuid` int(255) NOT NULL,
  `user_uuid` int(255) NOT NULL,
  `comment` varchar(250) NOT NULL,
  `created` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

我的用户表如下：

CREATE TABLE `users` (
  `uuid` int(255) NOT NULL,
  `username` varchar(15) COLLATE utf8_unicode_ci NOT NULL,
  `password` varchar(64) COLLATE utf8_unicode_ci NOT NULL,
  `foreName` varchar(30) COLLATE utf8_unicode_ci NOT NULL,
  `surName` varchar(30) COLLATE utf8_unicode_ci NOT NULL,
  `email` text COLLATE utf8_unicode_ci NOT NULL,
  `number` int(11) NOT NULL,
  `created` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `is_admin` tinyint(1) NOT NULL DEFAULT '0'
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

这是我当前的SQL：

SELECT
b.uuid AS 'Blog ID', b.title AS 'Blog Title', b.detail AS 'Blog Body', u.uuid AS 'Blog Author ID', u.username AS 'Blog Author Username',
(SELECT bc.comment WHERE u.uuid = bc.user_uuid) AS 'Comment',
(SELECT u.uuid WHERE u.uuid = bc.user_uuid) AS 'Comment Author ID',
(SELECT u.username WHERE u.uuid = bc.user_uuid) AS 'Comment Author Username' 
FROM blogs b
INNER JOIN blogs_comments bc ON b.uuid = bc.blog_uuid
INNER JOIN users u ON b.user_id = u.uuid

查询工作正常，但是它具有意外的输出，看起来像这样：

我希望输出将注释作者的所有注释放在第一行，然后每一行将特定于注释作者。 IE：

Comment Author    Comment        Comment
Jaquarh           TEST           Lorem Ipsum

任何想法都会非常有帮助，谢谢！

Answer 1

您需要每次都知道有多少条评论，但是如果您这样做，那么可能会有一个枢轴为您提供所需的信息。对于您的查询结构，我有些困惑，但我认为它可能与数据透视表类似：

SELECT
blog_id AS 'Blog ID',
title AS 'Blog Title',
detail AS 'Blog Body',
blog_author_id AS 'Blog Author ID',
username AS 'Blog Author Username',
Comment_Author_ID AS 'Comment Author ID',
Comment_Author_Username AS 'Comment Author Username',
[0] AS 'Comment 1', 
[1] AS 'Comment 2', 
[2] AS 'Comment 3', 
[3] AS 'Comment 4', 
[4] AS 'Comment 5', 
[5] AS 'Comment 6'

FROM
  (SELECT --A1
  b.uuid as blog_id, b.title, b.detail, u.uuid as blog_author_id, u.username, 
  (SELECT bc.comment WHERE u.uuid = bc.user_uuid) AS Comment,
  (SELECT u.uuid WHERE u.uuid = bc.user_uuid) AS Comment_Author_ID,
  (SELECT u.username WHERE u.uuid = bc.user_uuid) AS Comment_Author_Username,
  row_number() over(partition by b.uuid order by bc.comment) AS row_num
  FROM blogs b
  INNER JOIN blogs_comments bc ON b.uuid = bc.blog_uuid 
  INNER JOIN users u ON b.user_id = u.uuid AND u.uuid = bc.user_uuid
  ) as A1
PIVOT
(max([comment]) FOR row_num in ([0], [1], [2], [3], [4], [5])) AS pvt

本文可能也会为您提供帮助：https://docs.microsoft.com/en-us/sql/t-sql/queries/from-using-pivot-and-unpivot?view=sql-server-2017