Return one SQL row per product with price and latest date

Question

I am not a SQL guy, I have used it in the past and rarely have an issue that cant be solved by google... however this time I need to ask the Community.

I have a database

Answer 1

One method uses the ROW_NUMBER windowed function.

My query uses a common table expression (CTE) to provide sample data. When using this technique you always need a CTE, or subquery. This is because values generated in the SELECT clause are not available to the WHERE clause. This is a consequence of something called the Logical Processing Order. In other words; SQL Server generates the row numbers after it has filtered the data. CTEs/Subqueries provide you with a second WHERE clause, that is actioned after the row numbers have been generated**.

-- Returning the most recent record from a transaction table.
WITH SampleDate AS
    (
        SELECT
            ROW_NUMBER() OVER (PARTITION BY ProdNo ORDER BY TransactionDate DESC) AS Rn,
            *
        FROM
            (
                VALUES                      
                    ('3STRFLEX', 13.02, '20162911', 'AWC '),
                    ('3STRFLEX', 15.02, '20162011', 'DWC '),
                    ('3STRFLEX', 15.02, '20160101', 'AWC '),
                    ('AFTV2'   , 35.49, '20162708', 'AWC '),
                    ('AFTV2'   , 29.99, '20160106', 'DWC '),
                    ('AFTV2'   , 29.99, '20160205', 'AWC ')
            ) AS x(ProdNo, Price, TransactionDate, PurchasedBy)
    )
SELECT
    *
FROM
    SampleDate
WHERE
    Rn = 1
;

** Actually this isn't entirely true. It is called the logical order for a reason. SQL Sever can/will exeucte your queries any way it sees fit. But however your query is physically executed it will respect the logical order.