Mysql查询选择行，或者匹配其他表上的行

Question

I've got two tables:

user
ID --- Name

posts
ID --- UserID --- Text --- Postdate

Now I want to select all users and every post they've made in the past 15 minutes. I also want to display every user that didn't didn't even make a post with the matching conditions at all.

I'm currently doing it this way:

$user_q = mysql_query("SELECT * FROM user");
while ($a = mysql_fetch_assoc($user_q))
{
     $post_q = mysql_query("SELECT * FROM posts WHERE Userid=".$a['ID']." AND Postdate >= DATE_SUB(NOW(), INTERVAL 15 MINUTE)");
     //Do anything with this information
}

Do you have any ideas how I can put all this information in just one query? Doing so many queries makes the server running very slow.

Answer 1

What you want is a left outer join:

select u.*, p.*
from users u left outer join
     posts p
     on u.id = p.userid and
        p.Postdate >= DATE_SUB(NOW(), INTERVAL 15 MINUTE);

The left outer join keeps all the rows in the first table. If nothing matches in the second table, using the condition, then the values are NULL for those fields.

EDIT:

If you want to limit this to 50 random users, you can do this at the users level:

select u.*, p.*
from (select u.*
      from users u
      order by rand()
      limit 50
     ) u left outer join
     posts p
     on u.id = p.userid and
        p.Postdate >= DATE_SUB(NOW(), INTERVAL 15 MINUTE);

The order by rand() makes them random. You could order by anything -- name, date created, whatever. You can even leave the order by out and just take 50 arbitrary rows returned by the database.