Subtract values from SELECT in SQL

Question

I have the following 2 queries.:

1. shipManagementInvoiceNetValue:

   SELECT IFNULL (SUM (shipManagementInvoice.netto ), 0) AS shipManagementInvoiceNetValue
   FROM tckopf AS shipManagementInvoice
   WHERE shipManagementInvoice.referenzid = 1
       AND shipManagementInvoice.btyp = 5
   

2. shipManagementCreditNoteNetValue:

   SELECT IFNULL (SUM (shipManagementCreditNote.netto), 0) AS shipManagementCreditNoteNetValue
   FROM tckopf AS shipManagementCreditNote
   WHERE shipManagementCreditNote.referenzid = 1
       AND shipManagementCreditNote.btyp = 6
   

but I need the result shipManagementInvoiceNetValue - shipManagementCreditNoteNetValue. I'm using a HXTT JDBC DBF driver that supports more than SQL92.

How can I realize that?