How to delete duplicate row in mysql table based on the value of a third column in php -

i new php. asking delete row according value of third column. table consist of id , datetime fields. here code tried stuck next:

$cxn = mysqli_connect($host,$user,$pass,$dbname)     or die("couldn't connect server"); $qry = "select id, date(validdatetime) date, time(validdatetime) time tempstore order id"; $result = mysqli_query($cxn,$qry) or die("couldn't execute query");  while ($row=mysqli_fetch_assoc($result)) {     extract($row);     $hours = date("h",strtotime($time));     if ($id[$i] == $id[$i+1]) {         echo "<p>$id $date $hours</p>\n";     }     $i++;     //print_r($hours); } 

i extract date , time datetime fields in table extract again hours time distinguish if user have inputs in same hour in same date , delete leaving his/her first inputs in different hours. hope can understand trying achieve , me solve problem. thank in advance.