just create relation on existing column laravel code example

Example 1: whereHas site:https://laravel.com/docs/

use Illuminate\Database\Eloquent\Builder;

// Retrieve posts with at least one comment containing words like code%...
$posts = Post::whereHas('comments', function (Builder $query) {
    $query->where('content', 'like', 'code%');
})->get();

// Retrieve posts with at least ten comments containing words like code%...
$posts = Post::whereHas('comments', function (Builder $query) {
    $query->where('content', 'like', 'code%');
}, '>=', 10)->get();

Example 2: laravel how to query belongsTo relationship

$movies = Movie::whereHas('director', function($q) {
    $q->where('name', 'great');
})->get();