groupBy and orderBy Laravel for getting higher value - php

Question

I have problem when grouping and ordering the result of my query.
here is my data
+--------+-----------+----------+--------+-------+
| id_krs | id_mhs_pt | id_kelas | status | nilai |
+--------+-----------+----------+--------+-------+
| 38 | 3 | 29 | B | A |
| 45 | 3 | 30 | B | A |
| 46 | 3 | 31 | B | C |
| 18393 | 3 | 31 | U | A |
+--------+-----------+----------+--------+-------+
here is my query
DB::table('krs')->select('id_krs','id_mhs_pt','id_kelas','status','nilai')
->where('id_mhs_pt',3)->groupBy('id_kelas')->orderBy('nilai','asc')->get();
when i run the query, the result is:
+--------+-----------+----------+--------+-------+
| id_krs | id_mhs_pt | id_kelas | status | nilai |
+--------+-----------+----------+--------+-------+
| 38 | 3 | 29 | B | A |
| 45 | 3 | 30 | B | A |
| 46 | 3 | 31 | B | C |
+--------+-----------+----------+--------+-------+
and the result i want is
+--------+-----------+----------+--------+-------+
| id_krs | id_mhs_pt | id_kelas | status | nilai |
+--------+-----------+----------+--------+-------+
| 38 | 3 | 29 | B | A |
| 45 | 3 | 30 | B | A |
| 18393 | 3 | 31 | U | A |
+--------+-----------+----------+--------+-------+
Because the id_krs 18393 have greater nilai (score) than id_krs 46 (both using same id_kelas). I tried to change the order to asc or desc, but the result still the same. How to get that? Thank's!

Answer 1

The problem is that you loose the order on group by, so you must use a temporary table to get the minimum nilai for each id_kelas then join it in order to fetch the other columns:
DB::table('krs')
->select('id_krs', 'id_mhs_pt', 'id_kelas', 'status', 'nilai')
->leftJoin(
DB::raw('(select id_krs, min(nilai) as m_nilai FROM krs GROUP BY id_kelas) tmp'), function ($join) {
$join
->on(DB::raw('tmp.id_krs'), '=', DB::raw('krs.id_krs'))
->on(DB::raw('tmp.m_nilai'), '=', DB::raw('krs.nilai'));
}
)
->where('id_mhs_pt', 3)
->orderBy('nilai', 'asc')
->get();