Nested while loops in mysql data retrieval

Question

I am using nested while loops in order to retrieve information from 2 different tables (students, grades where both of those tables are linked by primary key - "id")

I want it to output in the following format :

Student ID "123"

 - Grade 1 for ID "123"
 - Grade 2 for ID "123"

Student ID "555"

 - Grade 1 for ID "555"
 - Grade 2 for ID "555"

Student ID "666"

 - Grade 1 for ID "666"
 - Grade 2 for ID "666"

Currently it outputs like this:

Student ID "123"

 - Grade 1 for ID "123" 
 - Grade 2 for ID "123"
 - Grade 1 for ID "555"
 - Grade 2 for ID "555" 
 - Grade 1 for ID "666" 
 - Grade 2 for ID "666"

Student ID "555"
Student ID "666"

This is the PHP code

<?php
include ("../../php/account.php");

$dbh = mysql_connect ( $hostname, $username, $password )
                   or die ( "Unable to connect to MySQL database" );
mysql_select_db( $project );

$s = mysql_query("SELECT * FROM newstudent");
$r = mysql_query("SELECT * FROM grades");  

while($rows=mysql_fetch_array($s))
    {
    echo "Student ID: ".$rows['id']."<br/>";
    echo "First Name: ".$rows['firstname']."<br/>";
    echo "Last Name: ".$rows['lastname']."<br/>";
    echo "Email: ".$rows['email']."<br/>";
    echo "<br/>";

            while($rows=mysql_fetch_array($r))
            {
                echo "Subject: ".$rows['subject']."<br/>";
                echo "Grade One: ".$rows['gradeone']."<br/>";
                echo "Grade Two: ".$rows['gradetwo']."<br/>";
                echo "Grade Three: ".$rows['gradethree']."<br/>";
                echo "<br/>";                
            }

    }

?>

If anyone knows a solution please help me out! many thanks!

Answer 1

Don't use nested loops, use a JOIN to get all the results in one query:

$q = mysql_query("SELECT * FROM newstudent s
                  JOIN grades g ON s.id = g.student_id
                  ORDER BY s.id") or die (mysql_error());

$last_student = null;
while ($row = mysql_fetch_assoc($q)) {
    if ($row['id'] !== $last_student) {
        $last_student = $row['id'];
        echo "Student ID: ".$row['id']."<br/>";
        echo "First Name: ".$row['firstname']."<br/>";
        echo "Last Name: ".$row['lastname']."<br/>";
        echo "Email: ".$row['email']."<br/>";
        echo "<br/>";
    }
    echo "Subject: ".$row['subject']."<br/>";
    echo "Grade One: ".$row['gradeone']."<br/>";
    echo "Grade Two: ".$row['gradetwo']."<br/>";
    echo "Grade Three: ".$row['gradethree']."<br/>";
    echo "<br/>";                
}

Answer 2

您正在覆盖$rows变量。

Answer 3

you are overwriting the $rows variable in the second loop, i think. try using 2 different variables

edit: The inner loop simply states "print all the grades" , so it has no relation with the student you are examining in the outer loop.

There is surely the needing of a foreign key that allows you to take the grades of only the student you are taking from the first loop, as someone else noticed.

Answer 4

Put your grades query inside your students loop and assign the student ID to it, also don't reassign the $rows variable inside your loops.

$s = mysql_query("SELECT * FROM newstudent");


while($rows=mysql_fetch_array($s))
    {
    echo "Student ID: ".$rows['id']."<br/>";
    echo "First Name: ".$rows['firstname']."<br/>";
    echo "Last Name: ".$rows['lastname']."<br/>";
    echo "Email: ".$rows['email']."<br/>";
    echo "<br/>";

    $r = mysql_query("SELECT * FROM grades WHERE studentID = '" . $row['id'] . "'");  

            while($r=mysql_fetch_array($r))
            {
                echo "Subject: ".$r['subject']."<br/>";
                echo "Grade One: ".$r['gradeone']."<br/>";
                echo "Grade Two: ".$r['gradetwo']."<br/>";
                echo "Grade Three: ".$r['gradethree']."<br/>";
                echo "<br/>";                
            }

    }

Answer 5

Try this

<?php
include ("../../php/account.php");

$dbh = mysql_connect ( $hostname, $username, $password )
                   or die ( "Unable to connect to MySQL database" );
mysql_select_db( $project );

$s = mysql_query("SELECT * FROM newstudent");

while($rows=mysql_fetch_array($s))
    {
    echo "Student ID: ".$rows['id']."<br/>";
    echo "First Name: ".$rows['firstname']."<br/>";
    echo "Last Name: ".$rows['lastname']."<br/>";
    echo "Email: ".$rows['email']."<br/>";
    echo "<br/>";

$r = mysql_query("SELECT * FROM grades");  

            while($rows1=mysql_fetch_array($r))
            {
                echo "Subject: ".$rows1['subject']."<br/>";
                echo "Grade One: ".$rows1['gradeone']."<br/>";
                echo "Grade Two: ".$rows1['gradetwo']."<br/>";
                echo "Grade Three: ".$rows1['gradethree']."<br/>";
                echo "<br/>";                
            }

    }

?>

Here You are overriding the $rows variable.So try to use different variable names.