Updating the intermediary table Best practice PHP, MySQL - php

Question

Lets Say I have Many to many relationship between the two tables students and subjects. So we create an intermediary table for mapping the data. Lets say it is students_subjects. The students_subjects table has 2 columns that is student_id & subject_id.
+------------+--------------+
| student_id | student_name |
+------------+--------------+
| | |
+------------+--------------+
+------------+--------------+
| subject_id | subject_name |
+------------+--------------+
| | |
+------------+--------------+
+------------+------------+
| student_id | subject_id |
+------------+------------+
| | |
+------------+------------+
Until a recent time when there is a update in students_subjects (student selecting subjects) what I did was delete all the subjects that student had an insert the new ones.
+------------+------------+----------+
| student_id | subject_id | selected |
+------------+------------+----------+
| 1 | 2 | 1 |
+------------+------------+----------+
| 1 | 2 | 0 |
+------------+------------+----------+
But recently I added a new column to this intermediary table as selected. So if a student selected new subjects and deselect some old subjects I add active for new ones and non_active for deselect ones. (I do this in PHP by getting the newly selected subjects and previously had subjects. Then compare both. When a student select subjects I check all the subjects that student has [active and non active]. If the newly selected subjects has previously non active subjects I active them and add the new ones. )
What is the best practice for doing this. I there a better way to achieve this?

Answer 1

I do think your first approach (deleting all associated rows and inserting new entries) is much more simpler.
If still want to have the selected field, a simple way would be to create unique composite index and using INSERT ... ON DUPLICATE KEY UPDATE. This can also prevent accidental insert of the same user_id and subject_id pair. First create the index:
ALTER TABLE `students_subjects` ADD UNIQUE `unique_student_subject`(`student_id`, `subject_id`);
Then, simply update the selected field for the student to 0 and insert the subjects the student selected. Example using PDO:
$studentId = 1;
$newlySelectedSubjects[] = ['subject_id' => 3];
$newlySelectedSubjects[] = ['subject_id' => 4];
$db = new PDO("connection string here", $username, $password);
$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$db->beginTransaction();
try{
$qry = 'UPDATE students_subjects SET selected = 0 WHERE student_id = :studentId';
$stmt = $db->prepare($qry);
$stmt->execute(['studentId' => $studentId]);
$insertQry = 'INSERT INTO students_subjects (student_id, subject_id, selected)'
.'VALUES (:studentId, :subjectId, 1) '
.'ON DUPLICATE KEY UPDATE '
.'selected = 1';
$insertStmt = $db->prepare($insertQry);
foreach ($newlySelectedSubjects as $subject) {
$params = ['studentId' => $studentId, 'subjectId' => $subject['subject_id']];
$insertStmt->execute($params);
}
$db->commit();
} catch (PDOException $e) {
//do something with exception
$db->rollBack();
}
What will happen is, if the student with student_id = 1 and subject_id = 3 already exist in the table, the selected field will be set to 1, if not a new record will be inserted.
If you require to track the user subjects selection and the de-selection, I suggest implementing some sort of audit trails and leave the students_subjects to only hold the relationship between students and subjects. It can be easier with the help of mysql TRIGGER.A simplified example of an audit trail table:
+------------+------------+----------+---------------------+
| student_id | subject_id | action | action_datetime |
+------------+------------+----------+---------------------+
| 1 | 2 | remove | 2018-01-01 00:01:23 |
+------------+------------+----------+---------------------+
| 1 | 4 | add | 2018-01-01 01:07:45 |
+------------+------------+----------+---------------------+