Getting the value of count(salesId) ?

I want to get the salesID of the count of sales order that uses the packing slip SDN001. 

Example:

Sales orders: SO001, SO002

Packing Slip Id: SDN001 

select count (salesId) from custPackingSlipTrans where custPackingSlipTrans.packingSlipId == 'SDN001'.

infolog(strFmt("%1", custPackingSlipTrans.salesId));

*This returns 2. 

Is it possible to get the sales Id value of the count(salesId), ?