public function where($field, $table, $value) {
        static $i = 0;
        $sql = self::resolve_field($field, $table) . ' ';

        if ($value === null) {
            $sql .= 'IS NULL';

        } else {
            $param = 'where' . $i++;
            $sql .= " = :$param";
            $this->params[$param] = $value;
        }

        $this->where[] = $sql;
    }