Inserting Data to a Table Retrieved from Two Merged Tables - php

Question

I have two courses. In each course there are several assignments. A student can follow both courses.
I have the following tables:courses,student_courses,course1
My courses table has st_index,assignmnet_no,marks,course_no as the fields
and student_courses table has st_index,course1,course2 where 1is inserted if the student follows course1 or course2 .
Table course1 has Student_Index, Assignment1, Assignment2,Assignment3 as fields.
I have inserted my data into tables courses,student_courses. By retrieving data from these I want to come up with course1 table.course1
should look as follows:
Here's my code:
if($_SESSION['user']['course']=="Course1"){
$result=mysql_query("SELECT student_index FROM student_courses WHERE course1=1");
while($index=mysql_fetch_array($result)){
echo $index[0];
echo"<br>";
$result2=mysql_query("SELECT * FROM courses WHERE course_no='Course1' AND st_index='$index[0]'");
while($i2=mysql_fetch_array($result2)){
$ass_no=$i2['assignment_no'];
if($ass_no=='1'){
echo $index[0];
$result3=mysql_query("INSERT INTO course1(Student_Index,Assignment1) VALUES('$index[0]','$i2[marks]')");
}
if($ass_no=='2'){
echo $index[0];
$result4=mysql_query("INSERT INTO course1(Assignment2) VALUES('$i2[marks]')");
}
if($ass_no=='3'){
$result5=mysql_query("INSERT INTO course1(Assignment3) VALUES('$i2[marks]')");
}
}
}
}
?>
The problem is for course1 only the first student_index assignment values gets inserted. In there also for the same student_index the values gets inserted into two separate rows. What I am trying to do is when we consider a row in course1, which is identified by student_index that row should have all the assignment marks of that student. How can I achieve this?
That is say I have two records that match the conditions for the table course1 as 945,568.
In the course1 it only gets inserted records for 965 and it also looks like below. . The two rows should be one, under same index_no
In order to check what is going wrong inside first while loop I used echo $index[0] . It prints 945,568 both. But in the if statements inside the other while loop only 945 gets printed. That is when the loop is run for the second time I think the line $result2=mysql_query("SELECT * FROM courses WHERE course_no='Course1' AND st_index='$index[0]'"); doesn't get executed.

Answer 1

The problem is that in SQL, INSERT always adds a new row. You've inserted a row for each mark, but only provided a Student_Index for one of them (thus the empty indexes on subsequent rows). What you want is the UPDATE command.
So let's just use INSERT once to create a row for each student, just before the inner WHILE:
INSERT INTO course1(Student_Index) VALUES('$index[0]')
and then fill the fields by replacing your INSERT statements with (e.g. for assignment 1):
UPDATE course1 SET assignment1 = '$i2[marks]' WHERE Student_Index = '$index[0]'
Alternatively, you can accomplish this in SQL alone with the following:
CREATE TABLE course1
SELECT s1.st_index,
a1.marks AS Assignment1,
a2.marks AS Assignment2,
a3.marks AS Assignment3
FROM courses a1, courses a2, course a3
WHERE a1.course_no = 1
AND a2.course_no = 1
AND a3.course_no = 1
AND a1.assignment_no = 1
AND a2.assignment_no = 2
AND a3.assignment_no = 3
AND a1.st_index = a2.st_index
AND a1.st_index = a3.st_index;
We've joined the course table three times to select out the three rows where the three marks reside for with course_no=1 and constrained them to be the same student in all three rows. This will return one row for each student. Also, With this solution there is no need for a reference to the student_courses table at all since you can infer the enrollment of a student in a course by the presence of marks in the course table.