userdefined function cross apply all days into the future sql
USE Northwind
GO
SELECT o1.OrderID, o1.OrderDate, ca.OrderID AS NextOrder,
ca.OrderDate AS NextOrderDate, CustomerID,
DATEDIFF(DAY, o1.OrderDate,ca.OrderDate) DaysToNextOrder
FROM Orders AS o1
CROSS APPLY
(SELECT TOP 1 o.OrderDate, o.OrderID
FROM Orders AS o
WHERE o.customerID = o1.customerID
AND o.OrderID > o1.OrderID
ORDER BY OrderID) AS ca
ORDER BY CustomerID, o1.OrderID