Illuminate\Database\Query\Builder::selectRaw PHP Method

    public function selectRaw($expression, array $bindings = [])
    {
        $this->addSelect(new Expression($expression));
        if ($bindings) {
            $this->addBinding($bindings, 'select');
        }
        return $this;
    }

 private function getQuery()
 {
     $concat = [];
     foreach ($this->by as $id) {
         $concat[] = "ifnull(`{$id}`,'')";
     }
     $key = "concat(" . implode(",'-',", $concat) . ") as `__key`";
     $columns = implode(",", array_merge([$key], $this->sync));
     return $this->builder->selectRaw($columns);
 }