How to get current and last order from Northwind db using Correlated queries - sql

Question

Using the northwind db on mssql, i am trying to retrieve the customer's last two order dates and calculate the time between the two orders.
So something like
select c.CompanyName, o.OrderDate, o2.OrderDate,
DateDiff(d, o.OrderDate, o2.OrderDate) as TimeElapsed
unfortunately not sure how to construct it from there.
i have something like this but it's still wrong.
select c.CompanyName, o.OrderDate, o2.OrderDate,
DateDiff(d, o.OrderDate, o2.OrderDate) as TimeElapsed
from Orders o
INNER JOIN Customers ON c.CustomerID = o.CustomerID
INNER JOIN (
select OrderID, OrderDate
FROM Orders
order by OrderDate
OFFSET 1 ROWS
FETCH NEXT 1 ROW ONLY
) as o2 ON o.OrderID = o2.OrderID;
can anyone assist.
Thank you

Answer 1

Northwind has been obsolete for years; even AdventureWorks has been replaced. The following uses the latter but you should be able to easily translate it to your schema. Two different approaches. The last 2 select statements are used to verify the results. Notice that customer 30099 has only one order.
set nocount on;
with cte as (select SalesOrderID, OrderDate, CustomerID, row_number () over (partition by CustomerID order by OrderDate desc) as rn
from Sales.SalesOrderHeader)
select top 10 * from cte
where rn <= 2
order by CustomerID, rn;
with cte as (select SalesOrderID, OrderDate, CustomerID, row_number () over (partition by CustomerID order by OrderDate desc) as rn
from Sales.SalesOrderHeader)
select cte.CustomerID, min(cte.OrderDate) as mindate, max(cte.OrderDate),
case when min(cte.OrderDate) = max(cte.OrderDate) then cast(null as int)
else datediff(day, min(cte.OrderDate), max(cte.OrderDate)) end as dif
from cte
where rn <= 2
group by cte.CustomerID
order by CustomerID;
with cte as (select SalesOrderID, OrderDate, CustomerID, row_number () over (partition by CustomerID order by OrderDate desc) as rn
from Sales.SalesOrderHeader)
select cte.CustomerID, minr.OrderDate as mindate, cte.OrderDate as maxdate,
datediff(day, minr.OrderDate, cte.OrderDate) as dif
from cte left join cte as minr on cte.CustomerID = minr.CustomerID and minr.rn = 2
where cte.rn = 1
order by cte.CustomerID;
select top 2 CustomerID, OrderDate from Sales.SalesOrderHeader where CustomerID = 30118 order by OrderDate desc;
select top 2 CustomerID, OrderDate from Sales.SalesOrderHeader where CustomerID = 30099 order by OrderDate desc;