I would think you could do something like
$userids = select user_id from users_table;
for each $userid in $userids
update users_table set join_date = 'select min(post_date) from post_table where user_id=$userid' where user_id = $userid;
next
and if that doesn't work you could just select all the user_ids into a text file, and then create another query file with the middle query explicitly specified for each user_id and then run that query file.