Student Result Management System - SQLi

$name=$_POST["class_name"];
$id=$_POST["class_id"];

// validation
if (empty($name) or empty($id) or preg_match("/[a-z]/i",$id)) {
 if(empty($name))
 echo '<p class="error">Please enter class</p>';
 if(empty($id))
 echo '<p class="error">Please enter class id</p>';
 if(preg_match("/[a-z]/i",$id))
 echo '<p class="error">Please enter valid class id</p>

$name=$_POST['student_name'];
$rno=$_POST['roll_no'];
if(!isset($_POST['class_name']))
 $class_name=null;
else
 $class_name=$_POST['class_name'];

// validation
if (empty($name) or empty($rno) or empty($class_name) or preg_match("/[a-z]/i",$rno) or !preg_match("/^[a-zA-Z ]*$/",$name)) {
 if(empty($name))
 echo '<p class="error">Please enter name</p>';
 if(empty($class_name))
 echo '<p class="error">Please select your class</p>';
 if(empty($rno))
 echo '<p class="error">Please enter your roll number</p>';
 if(preg_match("/[a-z]/i",$rno))
 echo '<p class="error">Please enter valid roll number</p>';
 if (!preg_match("/^[a-zA-Z ]*$/",$name)) {
 echo '<p class="error">No numbers or symbols allowed in name</p>

if(!isset($_POST['class_name']))
 $class_name=null;
else
 $class_name=$_POST['class_name'];
...
// validation
if (empty($class_name) or empty($rno) or $p1>100 or $p2>100 or $p3>100 or $p4>100 or $p5>100 or $p1<0 or $p2<0 or $p3<0 or $p4<0 or $p5<0 ) {
 if(empty($class_name))
 echo '<p class="error">Please select class</p>';
 if(empty($rno))
 echo '<p class="error">Please enter roll number</p>';
 if(preg_match("/[a-z]/i",$rno))
 echo '<p class="error">Please enter valid roll number</p>';
 if(preg_match("/[a-z]/i",$marks))
 echo '<p class="error">Please enter valid marks</p>';
 if($p1>100 or $p2>100 or $p3>100 or $p4>100 or $p5>100 or $p1<0 or $p2<0 or $p3<0 or $p4<0 or $p5<0)
 echo '<p class="error">Please enter valid marks</p>