How to make a "distinct" join with MySQL

Question

I have two MySQL tables (product and price history) that I would like to join:

Product Table: Id = int Name = varchar Manufacturer = varchar UPC = varchar Date_added = datetime

Price_h table: Id = int Product_id = int Price = int Date = datetime

I can perform a simple Left Join:

SELECT Product.UPC, Product.Name, Price_h.Price, Price_h.Date
FROM Product
LEFT JOIN Price_h
ON Product.Id = Price_h.Product_id;

But as expected if I have more than one entry for a product in the price history table, I get one result for each historical price.

How can a structure a join that will only return one instance of each produce with only the newest entry from the price history table joined to it?

Answer 1

Try this:

SELECT Product.UPC, Product.Name, Price_h.Price, MAX(Price_h.Date)
 FROM Product
 INNER JOIN Price_h
   ON Product.Id = Price_h.Product_id
GROUP BY Product.UPC, Product.Name, Price_h.Price

Answer 2

SELECT Product.UPC, Product.Name, Price_h.Price, Price_h.Date
FROM Product
LEFT JOIN Price_h
ON (Product.Id = Price_h.Product_id AND Price_h.Date = 
  (SELECT MAX(Date) FROM Price_h ph1 WHERE ph1.Product_id = Product.Id));

Answer 3

Use:

   SELECT p.upc,
          p.name,
          ph.price,
          ph.date
     FROM PRODUCT p
LEFT JOIN PRICE_H ph ON ph.product_id = p.id
     JOIN (SELECT a.product_id, 
                  MAX(a.date) AS max_date
             FROM PRICE_H a
         GROUP BY a.product_id) x ON x.product_id = ph.product_id
                                 AND x.max_date = ph.date