sum method
SUM aggregate
Implementation
Future<double?> sum(String column) async {
final originalSelects = List<String>.from(_selects);
_selects.clear();
_selects.add('SUM($column) as sum_value');
final result = await first();
_selects.clear();
_selects.addAll(originalSelects);
return result?['sum_value']?.toDouble();
}