MYSQL Query to count words written

[grey_goose]

I've got a query written that more or less counts words, that I can execute on a per-user basis because it needs the userid in WHEREs.

What I'd like to do is be able to update the entire table and run it as a cron job -- but I can't figure out how to get the userid into the subquery.
X = userid
@thisuser = where I need the userids

SET @thisuser := X;
UPDATE user 
SET words = (SELECT SUM(words)
   FROM (SELECT LENGTH(p.pagetext) - LENGTH(REPLACE(p.pagetext, ' ', ''))+1 AS words
   FROM post as p
   WHERE userid = @thisuser
  UNION ALL
  SELECT LENGTH(gm.pagetext) - LENGTH(REPLACE(gm.pagetext, ' ', ''))+1 AS words
   FROM groupmessage as gm
   WHERE postuserid = @thisuser) a)
WHERE userid = @thisuser;

I'm not sure if I'm going about this the wrong way, or just can't figure out how to fetch the userid of the current row that the query is looped on.

Suggestions?