Question

I have a situation where I need select the current and previous order amount in a single row using a select statement.

Order Table:

Customer Id
OrderId
OrderDate
OrderAmount

Current Select Statement:

SELECT o.OrderId, o.OrderDate, o.OrderAmount, po.OrderAmount
FROM Order o
LEFT JOIN (
   SELECT TOP(1) so.OrderAmount
   FROM Order so
   WHERE so.CustomerId = o.CustomerId and so.OrderId <> o.OrderId
   ORDER BY so.OrderDate DESC
   ) po

The problem is that the “where” clause in the sub query is not allowed. Is there another method for getting this information.

This is actually a simplification of a more complex select (for a view) that requires data for financial reports for the current and previous reporting period.

Answer 1

You would need OUTER APPLY here.

Your WHERE clause doesn’t look right though. I’ve assume OrderDate is unique below in being able to determine “previous” row.

SELECT o.OrderId,
       o.OrderDate,
       o.OrderAmount,
       po.OrderAmount
FROM   [Order] o
       OUTER APPLY(SELECT TOP(1) so.OrderAmount
                   FROM   [Order] so
                   WHERE  so.CustomerId = o.CustomerId
                          AND so.OrderDate < o.OrderDate
                   ORDER  BY so.OrderDate DESC) po  

But you may well be better off left joining on ROW_NUMBER

;WITH Ord
     AS (SELECT OrderId,
                OrderDate,
                OrderAmount,
                CustomerId,
                ROW_NUMBER() OVER ( PARTITION BY CustomerId 
                                       ORDER BY OrderDate)  AS RN
         FROM   [Order])
SELECT o.OrderId,
       o.OrderDate,
       o.OrderAmount,
       po.OrderAmount
FROM   Ord o
       LEFT JOIN Ord po
         ON o.CustomerId = po.CustomerId
            AND o.RN = po.RN + 1