mysql随机取数据

$num=10;
$coun=$song->count();
$min=$song->min('id');
$con = $song->max('id');
if($coun<$num){
$num=$countcus;
}
$i=1;
$flag=0;
while($i<=$num){
$rundnum=rand($min, $con);
if($flag!=$rundnum){
if($addlink=$song->getById($rundnum)){
$addlinks[]=$addlink;
$flag=$rundnum;
$i++;
}
}
}