Php to calculate marks and award grades - php

Question

Just wondering what is the problem with my below php, I am trying to insert into two tables marks and results. For the first type of exams I want it to be inserted in the two tables. Second type of exams to be inserted to marks and then update results table, that is to take the marks of the previous exams and add together the assign grade. When I post the first marks, it goes well. Now the second marks is the there is problem; marks table will be inserted successfully but updating results table gives different results, result column either become 0 or 1, and grade remains the same. Can anyone study my codes below and tell me where there is problem please or any other way to do this! I will appreciated. Thank you.
<?php
include('../database/dbcon.php');
$tearcher_post_id = $_POST['tearcher_id'];
$year_id = $_POST['year_id'];
$term_id = $_POST['term_id'];
$subject_post_code = $_POST['subject_code'];
$student_no = $_POST['student_no'];
$exam_id = $_POST['exam_id'];
$class_id = $_POST['class_id'];
$marks = $_POST['marks'];
$grade = '';
if($marks >=0 && $marks<= 19){
$grade = 'E';
}else if($marks > 19 && $marks <= 29){
$grade = 'D-';
}else if($marks > 29 && $marks <=34){
$grade = 'D';
}else if($marks > 34 && $marks <=39){
$grade = 'D+';
}else if($marks > 39 && $marks <=44){
$grade = 'C-';
}else if($marks > 44 && $marks <= 49){
$grade = 'C';
}else if($marks > 49 && $marks <= 54){
$grade = 'C+';
}else if($marks > 54 && $marks <= 59){
$grade = 'B-';
}else if($marks > 59 && $marks <= 64){
$grade = 'B';
}else if($marks > 64 && $marks <= 69){
$grade = 'B+';
}else if($marks > 69 && $marks <= 79){
$grade = 'A-';
}else if($marks > 79 && $marks <= 100){
$grade = 'A';
}
$query_details = mysql_query(
"SELECT staff.staff_id, students.student_id, subjects.subject_id
FROM staff, students, subjects
WHERE staff.id_number='$tearcher_post_id'
AND students.admission_no='$student_no'
AND subjects.subject_code='$subject_post_code'")or die(mysql_error());
$row_details = mysql_fetch_array($query_details);
$num_details = mysql_num_rows($query_details);
if($num_details > 0){
$staff_id= $row_details['staff_id'];
$student_id= $row_details['student_id'];
$subject_id= $row_details['subject_id'];
$query_marks = mysql_query("SELECT *
FROM marks
WHERE student_id='$student_id'
AND year_id='$year_id'
AND exam_type_id='$exam_id'
AND subject_id='$subject_id'")or die(mysql_error());
$num_marks = mysql_num_rows($query_marks);
if($num_marks > 0){
echo 'marks_excist';
}else{
$query = mysql_query("INSERT
INTO marks
(student_id,
staff_id,
term_id,
year_id,
exam_type_id,
subject_id,
class_id,
marks)
values(
'$student_id',
'$staff_id',
'$term_id',
'$year_id',
'$exam_id',
'$subject_id',
'$class_id',
'$marks' )")or die(mysql_error());
if($query){
$query_results = mysql_query("SELECT *
FROM results
WHERE student_id='$student_id'
AND term_id='$term_id'
AND year_id='$year_id'
AND subject_id='$subject_id'")or die(mysql_error());
$row_results = mysql_fetch_array($query_results);
$num_results = mysql_num_rows($query_results);
if($num_results > 0){
$id= $row_results['result_id'];
$results = $row_results['results'];
$total = $results + $marks;
$grade = '';
if($total >=0 && $total<= 19){
$grade = 'E';
}else if($total > 19 && $total <= 29){
$grade = 'D-';
}else if($total > 29 && $total <=34){
$grade = 'D';
}else if($total > 34 && $total <=39){
$grade = 'D+';
}else if($total > 39 && $total <=44){
$grade = 'C-';
}else if($total > 44 && $total <= 49){
$grade = 'C';
}else if($total > 49 && $total <= 54){
$grade = 'C+';
}else if($total > 54 && $total <= 59){
$grade = 'B-';
}else if($total > 59 && $total <= 64){
$grade = 'B';
}else if($total > 64 && $total <= 69){
$grade = 'B+';
}else if($total > 69 && $total <= 79){
$grade = 'A-';
}else if($total > 79 && $total <= 100){
$grade = 'A';
}
mysql_query("UPDATE results
SET results = '$total' AND grade = '$grade'
WHERE result_id = '$id'") or die(mysql_error());
echo 'true_marks';
}else{
mysql_query("INSERT
INTO results
(student_id,
year_id,
class_id,
term_id,
subject_id,
results,
grade)
values(
'$student_id',
'$year_id',
'$class_id',
'$term_id',
'$subject_id',
'$marks',
'$grade' )")or die(mysql_error());
}
}else{
echo 'false_marks';
}
}
}else{
echo 'details_dont_excist';
}
?>