DELIMITER $$
CREATE OR REPLACE DEFINER=`vn`@`localhost` FUNCTION `vn`.`invoiceOutAmount`(vInvoiceRef VARCHAR(15))
	RETURNS decimal(10,2)
	NOT DETERMINISTIC
	READS SQL DATA
BEGIN
	DECLARE totalAmount DECIMAL(10,2);

	SELECT SUM(vat) INTO totalAmount
		FROM (	
			SELECT iot.vat
				FROM invoiceOutTax iot
					JOIN invoiceOut io ON io.id = iot.invoiceOutFk
				WHERE io.ref = vInvoiceRef
			UNION ALL 
			SELECT ioe.amount
				FROM invoiceOutExpense ioe
					JOIN invoiceOut io ON io.id = ioe.invoiceOutFk
				WHERE io.ref = vInvoiceRef
		) t1;

	RETURN totalAmount;
END$$
DELIMITER ;