mysql - SUM Union All 作为 select 语句上的子查询

Question

我有三个表:测验、评论、问题。 它们都有一个名为 activityID 的 activity 外键。

我想在单个查询中对每个表上的条目数量进行求和，为所有事件返回相应的总和。

这是我的尝试:

SELECT
    `activity`.`id` AS `activityID`,
    `activity`.`name`,
    (SELECT SUM(`count`) FROM (
        SELECT COUNT(*) AS `count`
        FROM `quiz` WHERE `quiz`.`activityID` = `activity`.`id`

        UNION ALL

        SELECT COUNT(*) AS `count`
        FROM `comment` WHERE `comment`.`activityID` = `activity`.`id`

        UNION ALL

        SELECT COUNT(*) AS `count`
        FROM `question` WHERE `question`.`activityID` = `activity`.`id`
    ) `union`) AS `interactions`
FROM
    `activity`
WHERE 1
    AND `activity`.`eventID` = 1234
GROUP BY `activity`.`id`
ORDER BY `interactions` DESC

它不起作用，因为在嵌套子查询中找不到activity.id。是否可以将此值添加到嵌套子查询范围？或者甚至更好，是否有一种更简单的方法来执行此操作，而无需运行三个单独的子查询并添加它们？

这是我迄今为止最好的解决方案(不使用UNION):

SELECT
    `activity`.`id` AS `activityID`,
    `activity`.`name`,
    ( SELECT COUNT(*) AS `count`
      FROM `quiz` WHERE `quiz`.`activityID` = `activity`.`id` ) +
    ( SELECT COUNT(*) AS `count`
      FROM `comment` WHERE `comment`.`activityID` = `activity`.`id` ) +
    ( SELECT COUNT(*) AS `count`
      FROM `question` WHERE `question`.`activityID` = `activity`.`id` ) AS `interactions`
FROM
    `activity`
WHERE 1
    AND `activity`.`eventID` = 1234
GROUP BY `activity`.`id`
ORDER BY `interactions` DESC

谢谢。

Answer 1

您可以连接所有表并获取计数。

SELECT
    `activity`.`id` AS `activityID`,
    `activity`.`name`,
    SUM(I.count) as interactions
FROM
    `activity` A INNER JOIN 
    (SELECT `quiz`.`activityID`, COUNT(*) AS `count`
        FROM `quiz` 
        GROUP BY `quiz`.`activityID`
        UNION ALL
        SELECT `comment`.`activityID`, COUNT(*) AS `count`
        FROM `comment`
        GROUP BY `comment`.`activityID`
        UNION ALL
        SELECT `question`.`activityID`, COUNT(*) AS `count`
        FROM `question`
        GROUP BY `question`.`activityID`
    ) interactioncounts I on A.id = I.activityID
WHERE 1
    AND `activity`.`eventID` = 1234
GROUP BY `activity`.`id`
ORDER BY `interactions` DESC