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