[Eloquent]Union()で重複行があるのでdistinctしたい ref: https://qiita.com/nitaking/items/82fe97fffee9bef14b49

file0.txt

$first = DB::table('users')
    ->whereNull('first_name')
    ->get();

$users = DB::table('users')
    ->whereNull('last_name')
    ->get()
    ->union($first);

file1.txt

$first = DB::table('users')
    ->whereNull('first_name');

$users = DB::table('users')
    ->whereNull('last_name')
    ->union($first)
    ->get();

UNION_ALLと同じ

$first = DB::table('users')
    ->whereNull('first_name')
    ->get();

$users = DB::table('users')
    ->whereNull('last_name')
    ->get()
    ->union($first);

UNIONになる

$first = DB::table('users')
    ->whereNull('first_name');

$users = DB::table('users')
    ->whereNull('last_name')
    ->union($first)
    ->get();