SELECT "CUST_CODE", "ORD_DATE", 
	"ORD_DATE" - FIRST_VALUE("ORD_DATE") OVER (
		PARTITION BY "CUST_CODE"
		ORDER BY "ORD_DATE"
	) "GAP_UNTIL_NEXT_ORDER"
FROM "ORDERS"
ORDER BY "GAP_UNTIL_NEXT_ORDER" DESC;