Use column of parent query in subquery of a subquery

问题

With this query:

SELECT
    `id`,
    `type`,
    `subtype`,
    `title`,
    `shortdesc`,
    (SELECT COUNT(*)
     FROM `story_comments`
     WHERE `parent_id` = t1.`id`) as comments,
    (SELECT
        (ROUND( (
            SELECT (SUM(`rating` * `count`) / SUM(`count`) ) ) * 2) ) / 2 as result
            FROM
                    (SELECT rating, COUNT(*) as count
                     FROM `story_ratings` WHERE `parent_id` = t1.`id`
                     GROUP BY rating) as val) as rating,
    `calls`,
    `user`
FROM
    `storys` t1
WHERE
    `open` = 1 AND
    `modremove` = 0 AND
    `modblock` = ''
ORDER BY
    `opening`
DESC LIMIT 16;

I get this error: #1054 - Unknown column 't1.id' in 'where clause', which is caused by the subquery in the subquery (subquery after FROM).

But the t1.id in the first subquery is working fine. Why cant I use it in the FROM-subquery? I also tried variables, which also didnt work:

SELECT @i := `id` id, `type`, `subtype`, `title`, `shortdesc`, (SELECT COUNT(*) FROM `story_comments` WHERE `parent_id` = t1.`id`) as comments, 

(SELECT (ROUND( (SELECT (SUM(`rating` * `count`) / SUM(`count`) ) ) * 2) ) / 2 as result FROM (SELECT rating, COUNT(*) as count FROM `story_ratings` WHERE `parent_id` = @i GROUP BY rating) as val) as rating,

`calls`, `user` FROM `storys` t1 WHERE `open` = 1 AND `modremove` = 0 AND `modblock` = '' ORDER BY `opening` DESC LIMIT 16;

With the @i variable, result returned NULL on every row, what is wrong.

回答1:

Wow. So many nested subqueries. Instead of nesting queries to the ends of the earth, use JOINs and aggregate your data to calculate what you need. I had to make some guesses about your table structures because you didn't supply them (something that you should always do when posting a database question).

SELECT
    S.id,
    S.type,
    S.subtype,
    S.title,
    S.shortdesc,
    COUNT(DISTINCT SC.id) AS comments,
    AVG(SR.rating) AS rating,
    calls,
    user
FROM
    Storys S  -- Storys isn't the plural of Story, BTW
LEFT OUTER JOIN Story_Comments SC ON SC.parent_id = S.id
LEFT OUTER JOIN Story_Ratings SR ON SR.parent_id = S.id
WHERE
    S.open = 1 AND
    S.modremove = 0 AND
    S.modblock = ''
GROUP BY
    S.id,
    S.type,
    S.subtype,
    S.title,
    S.shortdesc,
    S.calls,
    S.user
ORDER BY
    opening
DESC LIMIT 16;

I don't think that the "* 2 ... / 2" does what you think it does, based on the various parentheses and the rounding might not be right here depending on the data type of your rating column (again, no table structures, so I'm stuck guessing).

I don't have a MySQL server handy, but testing this code on SQL Server (adjusting for difference in the ROUND function) gave the same results as your first query.

来源：https://stackoverflow.com/questions/37166457/use-column-of-parent-query-in-subquery-of-a-subquery

标签

mysql

sql

variables