Why doesn't distinct work in query builder?

Question

Laravel

Tell me why distinct does not work in query builder? We need to get records with a unique value categories.name

There is a query like this:

$subCategories = DB::table('categories')
            ->select(
                'categories.parent_category_id',
                'categories.name',
                'categories.has_subcategories',
                'user_categories.user_id'
            )
            ->leftJoin('user_categories', 'categories.id', '=', 'user_categories.category_id')
            ->where('parent_category_id', $id)
            ->distinct()
            ->get()
            ->toArray();

Result:

"subCategories": [
            {
                "parent_category_id": 1,
                "name": "Mac",
                "has_subcategories": 0,
                "user_id": 148
            },
            {
                "parent_category_id": 1,
                "name": "Mac",
                "has_subcategories": 0,
                "user_id": 149
            },
            {
                "parent_category_id": 1,
                "name": "Mac",
                "has_subcategories": 0,
                "user_id": 151
            }
        ]

And it should be:

"subCategories": [
            {
                "parent_category_id": 1,
                "name": "Mac",
                "has_subcategories": 0,
                "user_id": 148
            }
        ]

Answer 1

Because distinct works on a combination of all selectable fields, and here the user_id is different