What's wrong with this MySQL query? SELECT * AS `x`, how to use x again later?

Question

The following MySQL query:

select `userID` as uID,
(select `siteID` from `users` where `userID` = uID) as `sID`,
from `actions`
where `sID` in (select `siteID` from `sites` where `foo` = "bar")
order by `timestamp` desc limit 100

…returns an error:

Unknown column 'sID' in 'IN/ALL/ANY subquery'

I don't understand what I'm doing wrong here. The sID thing is not supposed to be a column, but the 'alias' (what is this called?) I created by executing (select siteID from users where userID = uID) as sID. And it’s not even inside the IN subquery.

Any ideas?

---

Edit: @Roland: Thanks for your comment. I have three tables, actions, users and sites. The table actions contains a userID field, which corresponds to an entry in the users table. Every user in this table (users) has a siteID. I'm trying to select the latest actions from the actions table, and link them to the users and sites table to find out who performed those actions, and on which site. Hope that makes sense :)

Answer 1

You either need to enclose it into a subquery:

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

, or, better, rewrite it as a 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

Create the following indexes:

actions (timestamp)
users (userId)
sites (foo, siteID)