Combining Multiple MySQL JOIN Queries - php

Question

I have this query which echos IDs of assignments for classes which users are enrolled in.
$sql = $db->prepare("SELECT assignments.*, enrollments.course_id, enrollments.student_id
FROM assignments
LEFT JOIN enrollments
ON assignments.course_id = enrollments.course_id
LEFT JOIN completed
ON assignments.id != completed.assignment_id
WHERE enrollments.student_id = ?
ORDER BY assignments.id DESC LIMIT 10
");
$sql->execute(array($login_id));
while($row = $sql->fetch())
{
echo $row['id'];
}
What would be the best way to do yet another check where I see if the assignment has been marked as completed?
This means that it would also need to check the "completed" table and make sure there is no row where the $login_id and assignment.id are present together for any of the assignments selected.
Here's a query I have right now to find completed assignment IDs for a user logged in.
$sqlcomplete = $db->prepare("SELECT * FROM completed
INNER JOIN students ON completed.student_id = students.id
WHERE completed.student_id = ?
");
$sqlcomplete->execute(array($login_id));
while($row = $sqlcomplete->fetch(PDO::FETCH_ASSOC))
{
echo "<li>You have completed assignment with ID ".$row['assignment_id']."</li>";
}
I've tried to do a more complex JOIN but I can't seem to figure it out. I also considered simply creating an array of the IDs of the assignments which the user has completed by querying that database alone, and throwing that ID into the while check, but I feel like that is not the best or most efficient solution.

Answer 1

You can use a LEFT JOIN and when completed.assignment_id IS NULL then that means there was no match returned from the completed table.
SELECT assignments.*, enrollments.course_id, enrollments.student_id
FROM assignments
LEFT JOIN enrollments ON assignments.course_id = enrollments.course_id
LEFT JOIN completed ON assignments.id = completed.assignment_id
WHERE enrollments.student_id = ?
AND completed.assignment_id IS NULL
ORDER BY assignments.id DESC LIMIT 10