Joining two table and updating the result - php

Question

I am joining two table and I want to update all the rows.
<?php
include("connection/mysqlconnect.php");
$sql=" SELECT course.duration, course.id, students.ID
FROM course, students
where course.id=course_id and course.duration = '2'";
$result = $conn->query($sql);
$count=mysqli_num_rows($result);
if($count>=1)
{
while($row = mysqli_fetch_array($result)) {
$id = $row['ID'];
$stat = 'Active';
$year = '2nd Year';
$Graduated = 'Graduated';
$sql1 = "UPDATE students SET Year='$Graduated', Status='non-Active'
WHERE ID = '$id' and (status='$stat' and Year='$year')";
echo "$id</br>";
}
}
?>
I tried the Select Statement above in "Run SQL query" and it query the result i want. and I want to update all of the query, but I cant. I tried Putting echo under the update and it echo the ID's I need to update, but my update statement is not executing.

Answer 1

Instead of selecting all students then updating one by one, you can actually to this in one shot by joining both tables and updating it.
UPDATE students s
INNER JOIN course c ON c.id = s.course_id
SET s.Year = '$Graduated',
s.Status = 'non-Active'
WHERE c.duration = '2'
AND s.status = '$stat'
AND s.Year = '$year'
It must also be taken into consideration that the query above is vulnerable with sql injection. This article below will guide you how to prevent from it.
How to prevent SQL injection in PHP?

Answer 2

The issue with the first query is that there are two columns with the same name; ID. So referencing the ID from the row generates an error. Use alias to fix it as shown below. For a better performance use an inner join instead. You also forgot to run the update query again your database.
<?php
include("connection/mysqlconnect.php");
$sql=" SELECT course.duration, course.id as cID, students.ID as sID
FROM course JOIN students ON course.id=course_id
where course.duration = '2'";
$result = $conn->query($sql);
$count=mysqli_num_rows($result);
if($count>=1)
{
while($row = mysqli_fetch_array($result)) {
$id = $row['sID'];
$stat = 'Active';
$year = '2nd Year';
$Graduated = 'Graduated';
echo "Student ID to be updated: $id<br/>";
$sql1 = "UPDATE students SET Year='$Graduated', Status='non-Active'
WHERE ID = '$id' and (status='$stat' and Year='$year')";
//you have to execute the query for the update to be done.
if ($conn->query($sql1) === TRUE) {
echo "Record updated successfully ";
} else {
echo "Error updating record: " . $conn->error;
}
}
}
$conn->close();
?>