Du könntest sie zusammenjoinen:
SQL-Code:
SELECT
SUM(productbookingpos.quantity) AS InUpToNow, SUM(invoicepos.quantity) AS OutUpToNow
FROM
productbookingpos, invoicepos
WHERE
productbookingpos.prod_ID = 111 and invoicepos.prod_ID= productbookingpos.prod_ID
AND productbookingpos.invalid =0 AND invoicepos.invalid =0;