In PHP MySQL how to sum or not sum multiple columns?

Question

I have three tables this is my codes:

<table border='1'>
 <tr>
 <th>Subject</th>
 <th>Mark1</th>
 <th>Mark2</th>
 <th>Mark3</th>
 <th>Sum Marks</th>
 </tr>
<?php
 $query2 = "SELECT students_info.s_name, subjects.subject, marks.*
 FROM subjects
 RIGHT JOIN (students_info
 RIGHT JOIN marks ON students_info.s_id = marks.s_id
 ) ON subjects.sub_id = marks.sub_id
 WHERE students_info.s_id = '".$s_id."'";
 $result2=mysqli_query($link,$query2) OR die(mysqli_error($link)); 
 while ($row=mysqli_fetch_array($result2))
 {
 ?>
 <tr>
 <td><?php echo $row['subject'];?></td>
 <td><?php echo $row['n1'];?></td>
 <td><?php echo $row['n2'];?></td>
 <td><?php echo $row['n3'];?></td>
 <td><?php echo $row['n1']+$row['n2']+$row['n3'];?></td>
 </tr>
<?php
 }
?>
 </table>
 <?php
}
?>

Output my codes:

enter image description here

I want when each mark form(mark1,mark2,mark3) for every subject less than 49 the sum operation not work in the cell sum marks appear empty value or empty cell, Please review my code and give me a help, to do work this code.

Answer 1

Could add them up in SQL by reverting to 0 for NULL columns like:

SELECT students_info.s_name, subjects.subject, marks.*, 
 COALESCE(marks.n1, 0) + COALESCE(marks.n2, 0) +COALESCE(marks.n3, 0) as sum_marks
FROM ....

• Talib: Thanks for your answer @danblack, i used your code but the same output appear, i want to when each mark less than(<49) the cell Sum Marks must be empty for this row
• danblack: I don't quite get the criteria however use: IF( expr < 49, 0, val).
• Talib: I used this criteria, but this way save only makr3 :( .
• danblack: IF( marks.n1 < 49 AND marks.n2 < 49 AND marks.n3 < 49, NULL, COALESCE(marks.n1, 0) + COALESCE(marks.n2, 0) +COALESCE(marks.n3, 0))
• Talib: Thanks @danblack, that's worked for me, but if the name subjects on the header name students on the left and marks on the center how this code work for sum?