public function innerJoin(string $table, $firstTableField, string $operator = null, string $secondTableField = null): Select
    {
        $this->joins[] = [
          'type' => 'INNER JOIN',
          'table' => $table,
          'first' => $firstTableField,
          'operator' => $operator,
          'second' => $secondTableField
        ];
        return $this;
    }