SQL Query to get First post of users

[induslady]

Hello,

Am using the below query to get the post of the users registered during Jan 2015:

SELECT a.title, a.username, a.postid FROM post a, user b 
WHERE a.userid = b.userid 
AND FROM_UNIXTIME (b.joindate) BETWEEN '2015-01-01' AND '2015-02-01'

From the above query result I would like to get ONLY the first post of each of these users. How do I write the query? Thanks in advance.