Mysql query to select rows and alternatively matching rows on other table -

i've got 2 tables:

user
id --- name

posts
id --- userid --- text --- postdate

now want select users , every post they've made in past 15 minutes. want display every user didn't didn't make post matching conditions @ all.

i'm doing way:

$user_q = mysql_query("select * user"); while ($a = mysql_fetch_assoc($user_q)) {      $post_q = mysql_query("select * posts userid=".$a['id']." , postdate >= date_sub(now(), interval 15 minute)");      //do information } 

do have ideas how can put information in 1 query? doing many queries makes server running slow.

what want left outer join:

select u.*, p.* users u left outer join      posts p      on u.id = p.userid ,         p.postdate >= date_sub(now(), interval 15 minute); 

the left outer join keeps rows in first table. if nothing matches in second table, using condition, values null fields.

edit:

if want limit 50 random users, can @ users level:

select u.*, p.* (select u.*       users u       order rand()       limit 50      ) u left outer join      posts p      on u.id = p.userid ,         p.postdate >= date_sub(now(), interval 15 minute); 

the order rand() makes them random. order -- name, date created, whatever. can leave order by out , take 50 arbitrary rows returned database.