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

You're on the right track. What if you use:

select t.ProdNo, t.TransactionDate as 'LastPurchaseDate', t.Price
from Transactions t
inner join (
     select ProdNo, max(TransactionDate) as 'LastPurchaseDate'
     from Transactions
     WHERE Price > 0
     group by ProdNo
 ) tm on t.ProdNo = tm.ProdNo and t.TransactionDate= tm.LastPurchaseDate

Note the change in join conditions.

What happened in your query: LastPurchaseDate = tm.LastPurchaseDate. There is only one column called LastPurchaseDate, so it's equating it with itself, which is always true. So you're left with t.ProdNo = tm.ProdNo, since t.ProdNo is not unique, you get multiple records returned for each t.ProdNo.