MySQL：使用别名定义 CONCAT 的数据类型

Question

This question is about the possibility to define a capacity/max-length when calling CONCAT and storing it as an alias.

I have a rather complex MySQL query using Common Table Expressions (CTE) used to model comments. It creates a new variable path consisting of a comment's score (# of votes) and id seperated by a comma and concatenated with its parent path using CONCAT . This allow sorting comments within threads by their score.

A path looks like eg 000010,000005,000014,000008 , which means that the comment with id 8 has a score of 14 and its parent, which itself does not have a parent, has id 5 and a score of 10 . With all comments having path of this format allows sorting them how I want.

The bottom line is that initially the path only consists of a single score with id, and in the recursive call we will continue concatenating into longer and longer paths when visiting the children.

However, it seems that the initial call to CONCAT immediately limits the size to 15 of all subsequent concatenations to the longest initial concatenation, so they are just cut after 15 characters. Making the initial concatenation longer than 15, will limit the subsequent concatenations to exactly the longest initial concatenation (so effectively doesn't concatenate anything).

Currently I have worked around this issue by initially padding a lot of zeros to the right and remove them in the recursive call. However, this uses regular expression and even though it is fairly simply I am afraid it is not good for performance.

Is there any way to define with the initial call of CONCAT what the capacity/maximum-length of the created alias variable should be?

This is the query that is made:

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

(Inspired by: Order comments by thread path and by number of total votes )

Initially I create path with CONCAT_WS(",", LPAD(r.votes,6,0), LPAD(r.id,6,0), LPAD(0,243,0)) as path , which creates the path containing the score and id of top-most comments (without parents), and pads 243 zeros to the right. So eg 000010,000005,0...0 for the top-most comment with id 5 .

Then recursively (but effectively only with the first recursive call, as thereafter the pattern never matches), we use regular expressions to remove all the trailing zeros including the last comma and add the score and id of this comment: CONCAT_WS(",", REGEXP_REPLACE(fle.path, ",[0]+$", ""), LPAD(r.votes,6,0), LPAD(r.id,6,0)) as path .

It would therefore be nice to just add something to the initial definition of path instead of this work-around. But I don't know what other way could be possible and better?

Any help and idea is appreciated!

// Edit: Problem was solved (and simplified) with GMB's help and a small addition, see my comment under the accepted answer.

Answer 1

How about appending the paths into a JSON array instead of a string? This seamlessly overcomes the problem that you are having, and you can still order by .

So:

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

Note that I simplified the query as follows:

• there is no need for a subquery in the anchor of the recursive query

• union all does not require parentheses around the two queries

Answer 2

Just for completion, the following is the code I came up with to use with MariaDB, for which the initial creation of the JSON array limits its capacity making it impossible to append to it.

Instead I use CONCAT_WS and CAST the initial creation of path to a VARCHAR(255) , which should be large enough to contain the longest paths.

I also had to change the calculation of the score in the path slightly, so that the order they appear is as expected.

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