Search in PurchaseOrder

Hi
I trying to create a SQL query that I can execute with 

var searchOptions = new OrderSearchOptions
{
CacheResults = false,
StartingRecord = 0,
RecordsToRetrieve = 10000,
Namespace = OrderNamespace.OrdersNamespace
};
var parameters = new OrderSearchParameters();
var sqlQueryFilter = new StringBuilder();
sqlQueryFilter.Append("OrderGroupId IN (SELECT ObjectId From OrderFormEx ");
sqlQueryFilter.AppendFormat("WHERE OrderFormEx.MyField = '{0}' )", DataToFind);
parameters.SqlWhereClause = sqlQueryFilter.ToString();
OrderContext.Current.Search<PurchaseOrder>(parameters, options);

I need to query also a field in OrderFormEx.
Any idea how to create the sqlQueryFilter?

Thanks