MySQL Not in usage

SELECT * from Zan where UID does in (select UID from Zan where Zhongjiang!=0) group by UID ORDER by rand () limit 40

But this is a relatively inefficient implementation and is finding a better way to use it.

I think it's better to write separately than the two statements, find out all the UID first, and then use not in this query speed much faster

$sql = "Select uid from Zan where Zhongjiang! = ' 0 '";
$res = $dbs->query ($sql);
$uid = ";
while ($o = $dbs->fetch_object ($res)) {
$uid. = $o->uid. ', ';
}
$uid =substr ($uid, 0,strlen ($uid)-1);
$sql = "SELECT * from Zan where UID is not ' in ($uid) ' GROUP BY ' UID Order by rand () limit 40";
$res = $dbs->query ($sql);
$arr =array ();
while ($o = $dbs->fetch_object ($res)) {
$arr []= $o;
}