我认为你可以这样做:
SELECT
u.id AS user_id,
CURRENT_TIMESTAMP AS granted_at,
(SELECT id FROM posts WHERE topic_id = YOUR_TOPIC_ID AND post_number = 1) AS post_id
FROM users u
INNER JOIN group_users gu ON gu.user_id = u.id
INNER JOIN groups g ON g.id = gu.group_id
WHERE g.name = 'beta-testers'
将 YOUR_TOPIC_ID 替换为实际值。
另外,我认为需要勾选「查询目标为帖子」选项。
如果可行请告诉我。