Now, as we have our new database structure, we definitely have to rewrite the SQL queries for student result fetching.

Start from what your user will type at the front-end. We can safely assume that your user will know at least his/her Roll Id, Class Name and Section. But our user doesn't know that how the class name and section is stored in the database. Also, we as developers don't know that how users will type his/her class name and section because there are many possibilities.

So, to make it clear for both the user and you, we should give them a drop-down menu of all the classes in our database. From which table of our database we will do it? Yes, you are right from tblclasses table. User will see his/her class name along with section and in return we will store the class id.

Here's the code for your reference.

<select name="class" class="form-control" id="default" required="required">
<option value="">Select Class</option>
<?php $sql = "SELECT * from tblclasses";
$query = $dbh->prepare($sql);
$query->execute();
$results=$query->fetchAll(PDO::FETCH_OBJ);
if($query->rowCount() > 0) {
foreach($results as $result) { ?>
<option value="<?php echo htmlentities($result->id); ?>"><?php echo htmlentities($result->ClassName); ?>&nbsp; Section-<?php echo htmlentities($result->Section); ?>
</option>
<?php }} ?>
</select>

Store the roll id and class id in some variables on the redirected page after user click submit.

$rollid=$_POST['rollid'];
$classid=$_POST['class'];
$_SESSION['rollid']=$rollid;
$_SESSION['classid']=$classid;

Now, coming to the query part where we bind our above parameters and fetch student result. Result should at least contains his/her subject name (not subject id because user don't know about subject id at all), corresponding marks.

Here's the query for the same.

"select t.StudentName, t.RollId, t.ClassId, t.marks, SubjectId, tblsubjects.SubjectName from 
(select sts.StudentName, sts.RollId, sts.ClassId, tr.marks, SubjectId from 
tblstudents as sts join tblresult as tr on tr.StudentId=sts.StudentId) as t 
join tblsubjects on tblsubjects.id=t.SubjectId where (t.RollId=:rollid and t.ClassId=:classid)"