getting mysql data for different users

If i have two users one users have many posts in post table i.e

postid, post, userid, time(i.e timestamp) 

and I have another table for friends i.e

id, friendid, userid, friend_sinc(i.e timestamp)

Now I want that users friend should see the posts o开发者_运维知识库f user1 after the date on which he added the user1 as friend. Not the posts older than the date on he added him as friend. How can I done it in single MySQL query?

This would look up all posts from people after they became friends with user 12:

select     *
from       Posts p
join       Friends f
on         p.userid = f.friendid
and        p.time >= f.friend_since
where      f.userid = 12

SELECT p.*
FROM post p, friend f
WHERE p.userid = f.userid AND p.time >= f.friend_sinc;

Edit: I guess it could be interpreted either way as which persond id is which, but the point is clear.

With a union

select p.* from posts p where p.userid = <my_user_id> 
union
select p.* from posts p, friends f where 
p.userid = f.friendid and 
f.userid = <my_user_id> and 
p.time >= f.friend_since

Or you consider yourself to be your own friend, that is, there is an entry in friend with friendid = <my_user_id> and userid = <my_user_id> and friend_since = <date user was created>, and then the second query works and return your own post as well. I don't think you get duplicates.