Laravel：查询where子句出错

Question

What I am trying to achieve is to allow teachers to import a student into different classes.

Note: A student can be multiple classes.

The problem is that when I show the list of students in a select dropdown it should return all students except for students that are not in this class (the class being the page that I am on, app.com/classes/5 for example).

$students = User::join('group_user', 'users.id', '=', 'group_user.user_id')
                ->role('student')
                ->where('group_user.group_id', '!=', $id)
                ->orderBy('users.name', 'asc')
                ->get();

This works and shows all students that are not in this specific class BUT if a student that's in this class and another class their name appears in the list and as duplicate names.

What can I do?

Answer 1

When MySQL's only_full_group_by mode is turned on, it means that strict ANSI SQL rules will apply when using GROUP BY

You should try to select fields from schema on which you can apply group by instead of select *.

$students = User::join('group_user', 'users.id', '=', 'group_user.user_id')
            ->role('student')
            ->where('group_user.group_id', '!=', $id)
            ->select('users.id', 'other fields you used')
            ->orderBy('users.name', 'ASC')
            ->groupBy('users.id')
            ->get();

Not IN is also useful in your case

User::select('fields you used')
    ->role('student')
    ->whereNotIn('id', DB::table('group_user')->where('group_id', $id)->pluck('user_id')) // $id = 5
    ->orderBy('name', 'ASC')
    ->get();

Answer 2

Modify your query to use distinct() like so;

    $students = User::join('group_user', 'users.id', '=', 'group_user.user_id')
                ->role('student')
                ->where('group_user.group_id', '!=', $id)
                ->orderBy('users.name', 'ASC')
                ->distinct()
                ->get();

You could also groupBy('users.id')

    $students = User::join('group_user', 'users.id', '=', 'group_user.user_id')
                ->role('student')
                ->where('group_user.group_id', '!=', $id)
                ->orderBy('users.name', 'ASC')
                ->groupBy('users.id')
                ->get();