SELECT y.id, y.yazar_name, JSON_OBJECTAGG(k.id, k.kitap_name) as kitaplar FROM yazarlar as y JOIN kitaplar as k ON y.id = k.yazar_id WHERE y.id = 1 GROUP BY y.id

$data = $st->fetch();
$yazarid = $data['id'];
$yazarname = $data['yazar_name'];
$kitaplar = json_decode($data['kitaplar'], true);
// ["1"=> "kitap_adi", "2"=> "kitap_adi_iki"] ..