Example SOQL to query for all fields using Dyanamic Query in Apex

query-by-sobject

// without an ID, simply specify the object to then derive the sobject type
DescribeSObjectResult describeResult = Account.getSObjectType().getDescribe();

List<String> fieldNames = new List<String>( describeResult.fields.getMap().keySet() );

String query =
  ' SELECT ' +
      String.join( fieldNames, ',' ) +
  ' FROM ' +
      describeResult.getName()
;

// return generic list of sobjects or typecast to expected type
List<SObject> records = Database.query( query );

System.debug( records );