如何与MySQL进行"独特"连接

Question

我有两个MySQL表(产品和价格历史记录),我想加入:

Product 表:

Id = int
Name = varchar
Manufacturer = varchar
UPC = varchar
Date_added = datetime

Price_h 表:

Id = int
Product_id = int
Price = int
Date = datetime

我可以执行一个简单的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;

但正如预期的那样,如果我在价格历史表中有多个产品条目,我会得到每个历史价格的一个结果.

如何构建一个只返回每个产品的一个实例的连接,只加入与它相关的价格历史表中的最新条目？

Answer 1

使用:

   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

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));