如何通过PHP查询并删除数据库多列重复数据？

<?php
$con = mysql_connect("localhost","root","");
if (!$con)
{
  die('Could not connect: ' . mysql_error());
}
$db_selected = mysql_select_db("test_db", $con);
if (!$db_selected)
{
  die ("Can/'t use test_db : " . mysql_error());
}
$sql = "SELECT * FROM friend";
$result=mysql_query($sql,$con);
while($myrow=mysql_fetch_row($result))
{
  $arr_data[$myrow[0]]=$myrow[1]."-".$myrow[2];
}
$arr_unique=array_unique($arr_data);
$arr_rep=array_diff_assoc($arr_data,$arr_unique);
//显示前后量
echo count($arr_data)."-".count($arr_unique);
foreach($arr_rep as $key=>$value){
 $sql_del = "DELETE FROM friend WHERE id = '{$key}' ";
 $result=mysql_query($sql_del,$con);
}
// 一些代码
mysql_close($con);
?>