Select statement returns empty field

I have a very simple select statement:

select firstonly RecId, OrderId, FeedType, TransactionRelation from transactionOverview
    where transactionOverview.OrderId == _orderId&& transactionOverview.FeedType == _feedType;

In this case i get the result without the TransactionRelation.

If i change the statement to select everything, there's the TransactionRelation with the expected value.

select * from transactionOverview
    where transactionOverview.OrderId == _orderId&& transactionOverview.FeedType == _feedType;

Interesting note:
This statement is inside a while loop. I used the second statement for the first records and it returned the expected value including TransactionRelation.

After this test i used the first select statement again and the first results from the select statement had the TransactionRelation with the expected value, but the third round in the while failed again (maybe because I hadn't selected this record prior with "select *"?).

Any ideas for this behaviour? Maybe I'm missing some important thing..