How to make a “distinct” join with MySQL

问题

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?

回答1:

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

回答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));

回答3:

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

回答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;

回答5:

Why not keep it simple and fast:

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

来源：https://stackoverflow.com/questions/3088686/how-to-make-a-distinct-join-with-mysql

标签

mysql

join

distinct

left-join

subquery