如何“独特”地加入MySQL

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

我有两个MySQL表(产品和价格历史)，我想加入:

Product table:

产品表:

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

Price_h table:

Price_h表:

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

I can perform a simple LEFT JOIN:

我可以执行一个简单的左连接:

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.

但正如预期的那样，如果在价格历史表中有一个以上的产品条目，那么每个历史价格将得到一个结果。

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?

如何构造一个连接，它只返回每个产品的一个实例，而只返回与它连接的价格表中最新的条目?

5 个解决方案

#1

Use:

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

#2

Try this:

试试这个:

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

#3

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));

#4

SELECT n.product_id, 
       n.product_name,
       n.product_articul,
       n.product_price,
       n.product_discount,
       n.product_description, 
       n.product_care,
       (SELECT photo_name FROM siamm_product_photos WHERE product_id = n.product_id LIMIT 1) AS photo_name
FROM siamm_product as n;

#5

Why not keep it simple and fast:

为什么不保持简单和快速:

SELECT 
 Product.UPC, Product.Name, Price_h.Price, Price_h.Date
FROM 
 Product
LEFT JOIN 
 Price_h
 ON Product.Id = Price_h.Product_id;
ORDER BY
 Price_h.Date DESC
LIMIT 1

#1