以数据库字段分组显示数据的sql语句(仔细讲解)

<?php
$dsn = "mysql:host=localhost;dbname=group";
$db = new PDO($dsn, 'root', '');
$db->exec('set names utf8');

$sql = "select conf_id from  `group` group by conf_id";
//$pre = $db->prepare($sql);
//$result_arr = array();
$pre = $db->query($sql);
$result_arr = $pre->fetchAll(PDO::FETCH_ASSOC);

//print_r ($result_arr);
foreach ($result_arr as $v)
{
 echo $v[conf_id]."<br>";
 $vv = $v[conf_id];
 $re = array();
 $sql1 = "select * from `group` where conf_id = '$v[conf_id]' order by time";
 $sql2 = "select * from `group` where conf_id = '$v[conf_id]' order by time DESC";
 $rs1 = $db->query($sql1);
 $rs2 = $db->query($sql2);
 $re1[$vv] = $rs1->fetchAll(PDO::FETCH_ASSOC);
 $re2[$vv] = $rs2->fetchAll(PDO::FETCH_ASSOC);
 print_r($re1[$vv][0]);
 print_r($re2[$vv][0]);
}