SQL query help needed

[doli]

I extracted number of users who have registerd on certain dates:

SELECT count(*) FROM `user` 
WHERE `joindate` >= UNIX_TIMESTAMP('2010-11-15 00:00:00')
AND `joindate` <UNIX_TIMESTAMP('2010-11-22 00:00:00')
AND `usergroupid` = 2

Now I want to check how many of these new users wrote a post within next 24 or 48 hours after registration, any idea how to do this? Probably some left join with post table ?

And one more question, what is 'dateline' in thread table?
Is this time when the first post was submited, or maybe last replay time ?

[Lynne]

thread.dateline is the first post time or thread creation time. thread.lastpost is the time of the last post.

I'll let someone else tackle the query question since I'm not very good at those.

[Lats]

Try this...

SELECT user.userid, user.username, FROM_UNIXTIME(post.dateline) post_date, post.postid  
FROM user, post 
WHERE joindate >= UNIX_TIMESTAMP('2010-11-15 00:00:00')
AND joindate <UNIX_TIMESTAMP('2010-11-22 00:00:00')
AND (post.dateline - user.joindate) < 172800
AND usergroupid = 2
GROUP BY
user.userid