Main query and subquery - subquery not returning desired results

Question

我有以下用于评论系统的SQL查询。

SELECT
    `main`.`comment_id`,
    `main`.`comment`,
    `main`.`timestamp`,
    `main`.`replay_comment_id`,
    COUNT(`replay`.`comment_id`) AS replays
FROM `posts_comments` AS `main`
LEFT JOIN `posts_comments` AS `replay` ON `replay`.`replay_comment_id` = `main`.`comment_id`
WHERE
    `main`.`post` = "107" AND (`main`.`replay_comment_id` IS NULL OR `main`.`comment_id` in ( SELECT
            `posts_comments`.`comment_id`
        FROM
            `posts_comments`
        WHERE
            `posts_comments`.`replay_comment_id` = `main`.`comment_id` ) )
GROUP BY
    `main`.`comment_id`
ORDER BY
    `main`.`comment_id` ASC;

使用以下数据库结构和内部值：

|------
|Column|Type|Null|Default
|------
|//**comment_id**//|int(10)|No|
|post|int(10)|No|
|author|int(10)|No|
|replay_comment_id|int(10)|Yes|NULL
|timestamp|int(10)|No|
|comment|varchar(200)|No|

== Dumping data for table posts_comments
|19|107|12|NULL|1688801931|comment 1
|20|107|12|NULL|1688801995|comment 2
|21|107|13|20|1688801995|test 1 comment for comment 2
|22|107|12|20|1688801995|test 2 comment for comment 2
|23|107|12|222|1688801995|test 1 comment for other comment

期望的结果将返回ID为19和20的评论，因为它们是主要评论，以及ID为21和22的评论，因为它们是ID为20的评论的子评论。不应返回ID为23的评论。现在的查询只返回主要评论。

在子查询中，如果我将main.comment_id替换为20，我可以得到我想要的结果，但是如果使用main.comment_id，则无法得到结果，我无法理解为什么。对于任何评论和想法，将不胜感激。

Answer 1

This method does not work. 19 and 20 satisfy main.replay_comment_id IS NULL, but in the subquery for 21, posts_comments.replay_comment_id = 19, this does not provide any results. Therefore, sub-comments 21 and 22 were not selected.

Please check the following query to see if it gives you results.

SELECT 
              p1.`comment_id`,
              p1.`comment`,
              p1.`timestamp`,
              p1.`replay_comment_id`,
              (
                CASE
                  WHEN p1.`replay_comment_id` IS NULL THEN 1 
                  WHEN p1.`replay_comment_id` IN (SELECT DISTINCT comment_id FROM posts_comments) THEN 1 
                  ELSE 0 
                END
              ) relationFlg,
              SUM(1- ISNULL(p2.`comment_id`)) hasChild 
            FROM
              `posts_comments` p1 
              LEFT JOIN `posts_comments` p2 
                ON p1.`comment_id` = p2.`replay_comment_id` 
            GROUP BY 1 
            HAVING relationFlg = 1 
            ORDER BY 1 ;