Question

我收到这个mysql错误，我不知道为什么。我正在关注一个教程，我已经从视频中逐字复制了语法。任何人都能发现错误吗？

这是错误：

您的SQL语法有错误;查看与您的MySQL服务器版本对应的手册，以便在第11行使用“LEFT JOIN（SELECT post_id，COUNT（comment_id）AS`total_co'附近使用正确的语法

     $query = "SELECT 
        `posts`.`post_id`                 AS `id`,
        `posts`.`post_title`              AS `title`,
         LEFT(`posts`. `post_body`, 512)  AS `preview`,
        `posts`.`post_user`               AS `user`,
         DATE_FORMAT(`posts`.`post_date`, '%d/%m/%Y %H:%i:%s') AS `date`,
         `comments`.`total_comments`,
        DATE_FORMAT(`comments`.`last_comment`, '%d/%m/%Y %H:%i:%s') AS `last_comment`

         FROM `posts`, 
         LEFT JOIN (
            SELECT 
                `post_id`,
                COUNT(`comment_id`) AS `total_comments`,
                MAX(`comment_date`) AS `last_comment`
            FROM `comments`
            GROUP BY `post_id` 
         ) AS `comments`
         ON `posts`.`post_id` = `comments`.`post_id` 
         ORDER BY `posts`.`post_date` DESC";

非常感谢您的帮助。谢谢。

Answer 1

删除第10行的逗号,：

FROM `posts`
     LEFT JOIN

正确的语法是：

FROM  
    <table a>
  LEFT JOIN  
    <table b>
      ON  <join condition>

与问题无关：

要提高效率（小），请将COUNT(comment_id)更改为：
```
        COUNT(*) AS total_comments,
```
您还可以在表(post_id, comment_date)上添加comments索引
在主SELECT列表中，如果您希望0 s NULL s而不发表评论的帖子，请将comments.total_comments,更改为：
```
COALESCE(comments.total_comments, 0) AS total_comments,
```

我认为与LEFT JOIN有关的SQL语法错误？

1 个答案: