Question

我有一个SQL查询，可以与博客作者一起拉出我所有的博客，并针对与该评论作者对应的每个博客评论执行此操作。

这3个表如下所示：

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;

ALTER TABLE `blogs`
    ADD PRIMARY KEY (`uuid`),
    ADD KEY `user_id` (`user_id`);

ALTER TABLE `blogs`
    MODIFY `uuid` int(255) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=3;

ALTER TABLE `blogs`
    ADD CONSTRAINT `blogs_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users` 
    (`uuid`) ON DELETE NO ACTION ON UPDATE NO ACTION;


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;

ALTER TABLE `blogs_comments`
    ADD PRIMARY KEY (`uuid`),
    ADD KEY `user_uuid` (`user_uuid`),
    ADD KEY `blog_uuid` (`blog_uuid`);

ALTER TABLE `blogs_comments`
    MODIFY `uuid` int(255) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=6;

ALTER TABLE `blogs_comments`
    ADD CONSTRAINT `blogs_comments_ibfk_1` FOREIGN KEY (`blog_uuid`) 
    REFERENCES `blogs` (`uuid`) ON DELETE NO ACTION ON UPDATE NO ACTION,
    ADD CONSTRAINT `blogs_comments_ibfk_2` FOREIGN KEY (`user_uuid`) 
    REFERENCES `users` (`uuid`) ON DELETE NO ACTION ON UPDATE NO ACTION;
COMMIT;

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;

ALTER TABLE `users`
    ADD PRIMARY KEY (`uuid`);

ALTER TABLE `users`
    MODIFY `uuid` int(255) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=8;

我当前的查询如下：

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

如果博客上有评论，此查询将正常工作。如果没有评论，则博客不返回。有谁知道我该如何改变这一点，以便即使没有评论也能发布博客？

预先感谢

更新：我的PDO如下：

$stmt = (new PDO('dsn', 'user', 'pswd'))->Prepare($sql);
$stmt->execute([], PDO::FETCH_ASSOC);
var_export($stmt->fetchAll()); // Array holds nothing for blogs with no comments

Answer 1

您需要一个left join，以users开头。但是，我认为您真正的困惑是您需要两次加入users：

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,
       bc.comment AS Comment,
       bc.user_uuid AS Comment_Author_ID,
       ubc.username AS Comment_Author_Username
FROM users u LEFT JOIN
     blogs b
     ON b.user_id = u.uuid LEFT JOIN
     blogs_comments bc
     ON b.uuid = bc.blog_uuid LEFT JOIN
     users ubc
     ON bc.user_uuid = u.uuid;

注意：

请勿对列别名使用单引号。仅对字符串和日期常量使用单引号。
不要为列别名提供需要转义的名称。我用下划线替换了空格。
如果您同时需要博客用户和评论用户的名称，则需要两次加入users。
我不明白您为什么在SELECT中使用没有FROM子句的子查询。

Answer 2

LEFT OUTER JOIN
    blogs_comments bc

应该这样做。

我的SQL仅在有注释的情况下带来结果

2 个答案: