MySQL: 用别名定义 CONCAT 的数据类型

Question

这个问题是关于在调用 CONCAT 并将其存储为别名时定义 capacity/max-length 的可能性。

我有一个相当复杂的 MySQL 查询，使用 Common Table Expressions (CTE) 来模拟评论。它创建一个新变量 path，由评论的分数（投票数）和 id 组成，用逗号分隔，并使用 CONCAT 与其 parent 路径连接。这允许按分数对主题中的评论进行排序。

A path 看起来像000010,000005,000014,000008，这意味着带有 id 8 的评论的得分为 14 而它的 parent 本身没有 parent，具有 id 5 和 10 的分数。所有具有这种格式 path 的评论都允许按我想要的方式对它们进行排序。

最重要的是，最初 path 只包含一个带有 id 的分数，在递归调用中，我们将在访问 children 时继续连接成越来越长的路径。

但是，似乎对 CONCAT 的初始调用立即将所有后续连接的大小限制为 15 到最长的初始连接，因此它们只是在 15 个字符后被剪切。使初始串联的长度超过 15，会将后续串联限制为恰好最长的初始串联（因此实际上不会串联任何东西）。

目前我已经解决了这个问题，方法是先在右边填充很多零，然后在递归调用中删除它们。但是，这使用了正则表达式，尽管它相当简单，但恐怕对性能不利。

有什么方法可以通过 CONCAT 的初始调用来定义创建的别名变量的 capacity/maximum-length 应该是什么？

这是进行的查询：

WITH RECURSIVE first_comments (id, content, parent_id, user_id, created, votes, path) AS (
        (
            SELECT r.id, r.content, r.parent_id, r.user_id, r.created, r.votes, CONCAT_WS(",", LPAD(r.votes,6,0), LPAD(r.id,6,0), LPAD(0,243,0)) as path
            FROM (
                SELECT c.id, c.content, c.parent_id, c.user_id, c.created, COUNT(DISTINCT v.id) AS votes
                FROM comments AS c
                LEFT JOIN comment_votes AS v ON c.id = v.comment_id
                WHERE c.post_id = ? AND c.parent_id IS NULL
                GROUP BY c.id
            ) as r
        )
UNION ALL
        (
            SELECT r.id, r.content, r.parent_id, r.user_id, r.created, r.votes, CONCAT_WS(",", REGEXP_REPLACE(fle.path, ",[0]+$", ""), LPAD(r.votes,6,0), LPAD(r.id,6,0)) as path
            FROM first_comments AS fle
                JOIN (
                    SELECT c.id, c.content, c.parent_id, c.user_id, c.created, COUNT(DISTINCT v.id) AS votes
                    FROM comments AS c
                    LEFT JOIN comment_votes AS v ON c.id = v.comment_id
                    WHERE c.post_id = ?
                    GROUP BY c.id
                ) AS r ON fle.id = r.parent_id
        )
    )
SELECT id, content, parent_id, user_id, path, created, votes FROM first_comments
ORDER BY pat

（灵感来自：Order comments by thread path and by number of total votes）

最初我使用 CONCAT_WS(",", LPAD(r.votes,6,0), LPAD(r.id,6,0), LPAD(0,243,0)) as path 创建 path，它创建包含 top-most 评论的分数和 ID 的路径（没有 parents），并填充 243 个零向右。所以例如000010,000005,0...0 用于 ID 为 5 的 top-most 评论。

然后递归地（但只对第一次递归调用有效，因为此后模式永远不会匹配），我们使用正则表达式删除所有尾随零，包括最后一个逗号，并添加此评论的分数和 ID：CONCAT_WS(",", REGEXP_REPLACE(fle.path, ",[0]+$", ""), LPAD(r.votes,6,0), LPAD(r.id,6,0)) as path.

因此，最好在 path 的初始定义中添加一些内容，而不是 work-around。但我不知道还有什么其他可能更好的方法？

感谢任何帮助和想法！

// 编辑：在 GMB 的帮助和少量补充下，问题已解决（并简化），请参阅已接受答案下的评论。

Answer 1

如何将路径附加到 JSON 数组而不是字符串？这无缝地克服了您遇到的问题，您仍然可以 order by.

所以：

WITH RECURSIVE first_comments (id, content, parent_id, user_id, created, votes, js_path) AS (
    SELECT 
        c.id, 
        c.content, 
        c.parent_id, 
        c.user_id, 
        c.created, 
        COUNT(DISTINCT v.id) AS votes,
        JSON_ARRAY(LPAD(COUNT(DISTINCT v.id), 6, 0), LPAD(c.id, 6, 0)) as js_path
    FROM comments AS c
    LEFT JOIN comment_votes AS v ON c.id = v.comment_id
    WHERE c.post_id = ? AND c.parent_id IS NULL
    GROUP BY c.id
    UNION ALL
    SELECT 
        r.id, 
        r.content, 
        r.parent_id, 
        r.user_id, 
        r.created, 
        r.votes, 
        JSON_ARRAY_APPEND(
            fle.js_path, 
            '$', LPAD(r.votes, 6, 0), 
            '$', LPAD(r.id, 6, 0)
        ) as js_path
    FROM first_comments AS fle
    JOIN (
        SELECT 
            c.id, 
            c.content, 
            c.parent_id, 
            c.user_id, 
            c.created, 
            COUNT(DISTINCT v.id) AS votes
        FROM comments AS c
        LEFT JOIN comment_votes AS v ON c.id = v.comment_id
        WHERE c.post_id = ?
        GROUP BY c.id
    ) AS r ON fle.id = r.parent_id
)
SELECT id, content, parent_id, user_id, js_path, created, votes 
FROM first_comments
ORDER BY js_path

请注意，我将查询简化如下：

递归查询的锚点中不需要子查询
union all 两个查询不需要括号

Answer 2

为了完成，以下是我想出的用于 MariaDB 的代码，JSON 数组的初始创建限制了它的容量，因此无法附加到它。

相反，我使用 CONCAT_WS 和 CAST 初始创建 path 到 VARCHAR(255)，它应该足够大以包含最长的路径。

我还必须稍微更改路径中分数的计算，以便它们出现的顺序符合预期。

WITH RECURSIVE first_comments (id, content, parent_id, user_id, created, level, votes, path) AS (
        SELECT
            c.id,
            c.content,
            c.parent_id,
            c.user_id,
            c.created,
            0 as level,
            COUNT(DISTINCT v.id) AS votes,
            CAST(CONCAT_WS(",", LPAD(999999-COUNT(DISTINCT v.id), 6, 0), LPAD(c.id, 6, 0)) AS VARCHAR(255)) as path
        FROM comments AS c
        LEFT JOIN comment_votes AS v ON c.id = v.comment_id
        WHERE c.post_id = ? AND c.parent_id IS NULL
        GROUP BY c.id
        UNION ALL
        SELECT
            r.id,
            r.content,
            r.parent_id,
            r.user_id,
            r.created,
            fle.level+1 as level,
            r.votes,
            CONCAT_WS(
                ",",
                fle.js_path,
                LPAD(999999-r.votes, 6, 0),
                LPAD(r.id, 6, 0)
            ) as path
        FROM first_comments AS fle
        JOIN (
            SELECT
                c.id,
                c.content,
                c.parent_id,
                c.user_id,
                c.created,
                COUNT(DISTINCT v.id) AS votes
            FROM comments AS c
            LEFT JOIN comment_votes AS v ON c.id = v.comment_id
            WHERE c.post_id = ?
            GROUP BY c.id
        ) AS r ON fle.id = r.parent_id
    )
    SELECT id, content, parent_id, user_id, created, level, votes, path
    FROM first_comments
    ORDER BY path ASC

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