group a query's values together based on its id - php

Question

I have data coming from the database , i am using JOIN(s) to select data from different tables,I need to be able to group the data together based on the specific id of the user in relation to the specific course, as an email needs to be sent based on the selection and within this selection(using a radio button) it will allow admin to be able to email all the users assigned to that specific category selected
i am currently getting duplicated data, like the users details will show on each category instead of being in one single category to pass into the array to email. I only need to select one radio button per category/course but now i am getting multiple radio buttons.
here is my query:
$query="SELECT * FROM course_student
JOIN course ON course.course_id=course_student.course_id
JOIN student ON student.student_id=course_student.student_id
WHERE course.course_id=course_student.course_id
ORDER BY course.course_id";
this is my loop to select data -- it's creating duplicate names for the entries where i just want one name with all of the data that is supposed to be in it
$result=mysqli_query($connection,$query);
confirmation($connection);
while($course_email_students = mysqli_fetch_assoc($result)){
$course_email = $course_email_students['student_email'];
$course_name = $course_email_students['course_name'] ."<br/>";
here is my html:
<input type="radio" name="course_mail[]" value="<?php echo $course_email ;?>">
<?php echo $course_name ?>
<?php } ?>
here is more code
html
<form action ="#" method="POST">
<P> <label for="">Send to specific student courses</label>
</p>
<?php // email specific students
$query = "SELECT student.student_email, course.course_name, course.course_id
FROM course_student
JOIN student ON student.student_id=course_student.student_id
JOIN course ON course.course_id=course_student.course_id
GROUP BY student.student_email, course.course_id
ORDER BY course.course_id";
$result=mysqli_query($connection,$query);
confirm_query($connection);
while($course_email_students=mysqli_fetch_assoc($result)){
$course_student_email=$course_email_students['student_email'];
$course_student_email_name=$course_email_students['course_name'] ."<br/>";
var_dump($course_email_students['student_email']);
?>
<input type="radio" name="course_email[]" value="<?php echo $course_student_email ;?>">
<?php echo $course_student_email_name ?>
<P> <label for="">Message</label>
<p><textarea rows="10" cols="20" name="message"></textarea></p>
</p>
<input type="submit" name="submit" value="send">
here is the php for testing to see what comes through
if(isset($_POST['submit'])){
// do validation
if(isset($_POST['course_email'])){
var_dump($_POST['course_email']);
}
}

Answer 1

From the PHP you've posted, it appears that you are just using data from two, possibly three columns in your query. If so, it is much more efficient to select the specific columns you're interested in in the SQL. There's also a redundancy in the query where you're using the same condition for a WHERE as you have in a JOIN:
$query="SELECT student.student_email, course.course_name, course.course_id FROM course_student
JOIN student ON student.student_id=course_student.student_id
JOIN course ON course.course_id=course_student.course_id <-- !!!
WHERE course.course_id=course_student.course_id <-- !!!
ORDER BY course.course_id";
There are two ways of selecting unique combinations of results with this select query; you can use DISTINCT or GROUP BY.
DISTINCT:
$query = "SELECT DISTINCT student.student_email, course.course_name, course.course_id
FROM course_student
JOIN student ON student.student_id=course_student.student_id
JOIN course ON course.course_id=course_student.course_id
ORDER BY course.course_name";
GROUP BY:
$query = "SELECT student.student_email, course.course_name, course.course_id, student.student_id
FROM course_student
JOIN student ON student.student_id=course_student.student_id
JOIN course ON course.course_id=course_student.course_id
GROUP BY student.student_email, course.course_name
ORDER BY course.course_name";
Now for the PHP code:
$c_name = ''; # course name
$c_id = ''; # course ID
$students = array();
while($aa = mysqli_fetch_assoc($result){
if ($aa['course_name'] !== $c_name) {
# the course name has changed. print out the course and student data
make_radios($c_name, $c_id, $students);
# set c_name to the new course name
$c_name = $aa['course.course_name'];
$c_id = $aa['course.course_id'];
# set the students to the new student
$students = array( $aa['student.student_email'] );
}
else {
# add this student to the list of students
$students[] = $aa['student.student_email'];
}
}
# print out the last set of data
make_radios( $c_name, $c_id, $students );
function make_radios( $course_name, $course_id, $email_arr ) {
$html = '<input type="radio" id="email'
. $course_id . '" name="course_email[]" value="'
. implode(',', $email_arr) . '"> <label for="email'
. $course_id . '">$course_name</label>';
# I don't know if you want to list all the email addresses or not... in case you do:
$html .= "<ul>";
foreach ($email_arr as $e) {
$html .= "<li>$e</li>\n";
}
$html .= "</ul>";
# append the message box
$html .= '<label for="message' . $course_id . '">Message</label>'
. '<textarea id="message' . $course_id . '" rows="10" cols="20" name="message">'
. "</textarea>\n";
echo $html; # or you could return $html
}
I would not recommend that you have the email addresses on the form--I'd use the student IDs and the students' names instead, and then have your script pull the appropriate email addresses from the database--but it's up to you, obviously.
Please read through the code and check you understand what it's doing. I'm happy to answer any questions.

Answer 2

Group and Order are words that have meaning both in English and in SQL jargon. You've said you want your values grouped. I think you mean, in SQL terms, you want them ordered, in such a way that each course's student emails are together.
There are some problems with your query.
First, Pro tip: Avoid using SELECT * in software. (It's OK when you're troubleshooting databases, but it's wasteful and confusing in software, especially when you're JOINing more than one table.)
Second, you have this condition repeated in both your JOIN...ON and your WHERE clause. That's redundant.
course.course_id = course_student.course_id
Leave it in your ON clause and out of your WHERE clause.
Third, your result set's order is underdetermined. This may not matter, but you are ordering only by course id. If you care whether your result set is ordered by student email, then mention it in the ORDER BY clause.
Fourth, you do have three tables. But one of them, course_student, appears to be a classic join table allowing there to be a many-to-many relationship between courses and students. So it seems likely you only have two tables with actual application data items in them.
Finally, from the code in your question it looks like you want
course_id
course_name
student_email
in your result set generated by your SELECT query. Your question's narrative mentions category, but because you've used SELECT * we can't tell what you mean by that.
To get the unique values of those three fields in your result set, use this query:
SELECT DISTINCT
course.course_id,
course.course_name,
student.student_email
FROM course_student
JOIN course ON course.course_id=course_student.course_id
JOIN student ON student.student_id=course_student.student_id
ORDER BY course.course_id, student.student_email
The DISTINCT qualifier eliminates duplicates.
From your comment, it's still hard to tell what you want. But it seems that you want to try this:
SELECT GROUP_CONCAT(DISTINCT course.course_name),
student.student_email
FROM course_student
JOIN course ON course.course_id=course_student.course_id
JOIN student ON student.student_id=course_student.student_id
GROUP BY student.student_email
ORDER BY student.student_email
Or maybe you want this.
SELECT GROUP_CONCAT(DISTINCT student.student_email),
course.course_name
FROM course_student
JOIN course ON course.course_id=course_student.course_id
JOIN student ON student.student_id=course_student.student_id
GROUP BY course.course_name
ORDER BY course.course_name
This second one is going to fail in your large courses because of a limitation of GROUP_CONCAT(). You may need to write application code to organize your web page course by course, or student by student.