operator [] method
The value for the given key, or null if key is not in the map.
Some maps allow null as a value.
For those maps, a lookup using this operator cannot distinguish between a
key not being in the map, and the key being there with a null value.
Methods like containsKey or putIfAbsent can be used if the distinction
is important.
Implementation
@override
MariaDbColumn operator [](Object? key) {
  return _columns.firstWhere(
    (element) => element.name == key,
    orElse: () => throw Exception(
      "This mariadb table `$name` has no column "
      "with the name `$key`",
    ),
  );
}