I recently wrote about the SQL to return the last record in SQL Server. Adam Butt of APB reports wrote to remind me that if you are using in SQL Server or Oracle you can do this more efficiently using a RowNumber “Over” a Partition. He also gave me an example which has helped me understand the pieces that go into this method. There are three components:
RowNumber() will number all the rows in your results, like a Crystal running total. However, it won’t work unless you give it an OVER clause to put the records in order. The OVER clause is just an “ORDER BY clause put in parentheses right after the word OVER. This statement would number all the records in your results from 1 to n:

ROW_NUMBER() OVER( ORDER BY Order.OrdDate DESC) AS RowNum

You add one more piece when you want the Row Number to start over with each group, the “Partition By”. Partition By is similar to Group By but happens inside the OVER clause. This statement would number the records in your results from 1 to n within each Customer:

ROW_NUMBER()
OVER( PARTITION BY Customer.Name ORDER BY Order.OrdDate DESC) AS RowNum

Notice that in both cases the ORDER BY is “DESC” or descending which means the latest record for each customer will be first or Row_Number 1. We can use that in the SQL below to make sure we only see the last record for each customer.

SELECT * FROM ( SELECT Order.OrdDate, Orders.OrderNo, Orders.Amount, Customer.Name,ROW_NUMBER() OVER(PARTITION BY Customer.Name ORDER BY Order.OrdDate DESC) AS RowNum FROM Orders Orders
INNER JOIN Customer Customer ON Orders.CustomerID=Customer.ID
) X
WHERE
X.RowNum= 1

By changing the final WHERE clause you can also do things like select the last 3 orders for each customer:

X.RowNum <= 3

And the OVER() can also be used without the row_number() to create running totals in your SQL results. For instance this expression would create a running sum of order amounts for each customer:

sum(orders.amount) OVER( PARTITION BY Customer.Name
ORDER BY Order.OrdDate DESC) AS RunningAmt