Can you suggest me the best way to get what I want from my database please?

Question

Hello. I am making a complex web app and I got stuck on one little problem. I need to knkw how to get certain input out of my database in a elegant way. I hav two tables there. One of them is named "classes" and has two columns: id and name. The second table is called "users" and has 3 columns: id, name, memberIn. memberIn is string that contains id's of classes the user has access to. Id's of classes are separated with commas (such as 1,2,4). I need to get names of the classes a user DOESN'T have access to.

Example:
users:
1 - John - 2,4

classes:
1 - 3.B
2 - 7.A
3 - 5.D
4 - 1.C

output for John: 3.B, 5.E.

I already have a code that works a bit, but it's unstable and too long and ugly.

Answer 1

I get ID's of classes from the memberIn column with this code:

/*Connecting to database and saving connection to variable $connection*/
$query = 'SELECT memberIn FROM users WHERE name="John"';
$result = mysqli_query($connection, $query);
$result = mysqli_fetch_array($result);
$result = $result['memberIn'];
$values = explode(',',$result);
/*$values now contains ID's of classes John is already member in.*/