Sottrarre i valori da selezionare in sql

Question

Ho le seguenti 2 domande.:

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
   

Ma ho bisogno del risultato shipManagementInvoiceNetValue - shipManagementCreditNoteNetValue. Sto usando un Driver HXTT JDBC DBF che supporta più di SQL92.

Come posso rendermi conto?