您要么需要将其包含在子查询中：
SELECT *
FROM (
SELECT userID as uID, (select siteID from users where userID = actions.userID) as sID,
FROM actions
) q
WHERE sID IN (select siteID from sites where foo = "bar")
ORDER BY
timestamp DESC
LIMIT 100
，或者，最好将其重写为 JOIN
SELECT a.userId, u.siteID
FROM actions a
JOIN users u
ON u.userID = a.userID
WHERE siteID IN
(
SELECT siteID
FROM sites
WHERE foo = 'bar'
)
ORDER BY
timestamp DESC
LIMIT 100
创建以下索引：
actions (timestamp)
users (userId)
sites (foo, siteID)
602392714
清零编程群