Soustraire les valeurs de Select in SQL

Question

J'ai les 2 requêtes suivantes.:

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
   

Mais j'ai besoin du résultat shipManagementInvoiceNetValue - shipManagementCreditNoteNetValue. J'utilise un HXTT JDBC DBF Pilote qui soutient plus que SQL92.

Comment puis-je réaliser cela?