如何打造“与众不同"的加入 MySQL [英] How to make a "distinct" join with MySQL

问题描述

我有两个想要加入的 MySQL 表(产品和价格历史记录):

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

产品表:

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:

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;

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，我会为每个历史价格获得一个结果.

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.

如何构造一个只返回每个产品的一个实例的连接结构，其中只连接价格历史表中的最新条目?

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?

推荐答案

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

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