I don’t know what might be possible with SQL queries, as there is no documentation yet. But there is no linkage of any kind between customers or suppliers and cash sales or purchases. You mentioned that you have a customer called Cash Sales. You may, in fact, have created one with that name in the
Customers tab. But entering that name in the Payer field of a receipt only brings up the name because of previous entries on other forms, not because of any recognition that this is a defined Customer in Manager’s context. You might as well enter Spiderman.
I think it would be a major enhancement if defined customers and suppliers were recognized when entering receipts and payments, allowing you to include not just credit sales/purchases on statements, but also cash transactions. Then you could get what you need just by defining one anonymous cash customer.