只有在有评论的情况下，我的SQL才会产生结果_Sql_Innodb

只有在有评论的情况下，我的SQL才会产生结果

sql

只有在有评论的情况下，我的SQL才会产生结果,sql,innodb,Sql,Innodb,我有一个SQL查询，它可以把我所有的博客都从博客作者那里拉出来，并且它会对每个与评论作者相对应的博客评论都这样做这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 D

我有一个SQL查询，它可以把我所有的博客都从博客作者那里拉出来，并且它会对每个与评论作者相对应的博客评论都这样做

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;

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

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

我当前的查询如下所示：

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;

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

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

如果博客上有评论，这个查询可以正常工作。如果没有评论，博客将不会返回。有没有人知道我该如何改变这一点，这样即使没有评论，它也会带来博客

提前谢谢

更新：我的PDO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;

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

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

应该这样做。

您需要一个

左连接

，从

用户开始。然而，我认为您真正的困惑在于您需要加入用户两次：
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:

不要对列别名使用单引号。仅对字符串和日期常量使用单引号
不要为列别名指定需要转义的名称。我用下划线替换了空格
如果您想同时获得博客用户和评论用户的姓名，那么您需要加入users
两次
我不明白为什么在SELECT
子句中使用子查询，而不使用FROM
子句
那太棒了！我真的很感激这些笔记，我所知道的一切都是自学的，所以这可能就是为什么我把它拿来做子查询的原因。谢谢没有索引？至少有一个主键
。UUID将限制可伸缩性。另请参见提示：每个uuid
都是带有auto_increment@RickJames的主键。出于某种原因，我的SQL转储没有添加这一点。我还设置了外键，我越来越糊涂了。我没有看到自动增量。UUID和auto_increment是两种不同的动物。可能在CREATE TABLE
之后不久就是ALTER TABLE。。添加主键..
？@RickJames刚刚选中，有。很抱歉没有提供这个部分，我只是从SQL导出中复制了create table
部分，假设它涵盖了所有内容。噢，我想看看键。