ScalarList selectList = new ScalarList();

selectList.Add(NW.Order.ShipVia);

selectList.Add(NW.Order.ShipRegion);

ScalarList groupByList = new ScalarList();

groupByList.Add(NW.Order.ShipRegion);

groupByList.Add(NW.Order.ShipVia);

SelectQuery query = OQL

  .Select(selectList)

  .From(NW.Order)

  .GroupBy(groupByList);

The SQL statements generated as follows (taking SQL Server as an example)

SELECT

  [Orders].[ShipVia] , [Orders].[ShipRegion]

FROM [Orders]

GROUP BY

  [Orders].[ShipRegion] , [Orders].[ShipVia]