如何通过MySQL查询从联接表中检索最低价格

Question

I have two tables

Products
id, name, last_updated
1, Computer table,  2014-07-20 09:00:00
2, Fruits basket,  2014-07-21 09:00:00
3, Hard Disk,  2014-07-22 09:00:00


Products_prices
id, product_id, min_price, max_price, description
1, 1, 10.00, 20.00, glass top
2, 1, 40.00, 80.00, hard wood
3, 1, 5.00, 10.00, pvc
4, 2, 15.00, 30.00, Mangoes only
5, 3, 30.00, 60.00, 1 TB
6, 3, 45.00, 90.00, 2 TB
7, 3, 20.00, 40.00, 500 GB

I need to retrieve information as below

product.id, product.name, product_prices.min_price, product_prices.max_price, price_diff_percentage ( calculated as round((max_price-min_price)/max_price*100) )

If I do a LEFT JOIN eg

SELECT *, round((max_price-min_price)/max_price*100) price_diff_pct 
FROM products p LEFT JOIN Product_prices pp on pp.product_id = p.id 

Then I get list of product with all the product prices rows eg

1, computer table, 10.00, 20.00, 50, glass top
1, computer table, 40.00, 80.00, 50, hard wood
1, computer table, 5.00, 10.00, 50, pvc
2, fruits basket, 15.00, 30.00, mangoes only
3, hard disk, 30.00, 60.00, 50 1 TB
3, hard disk, 45.00, 90.00, 50 2 TB
3, hard disk, 20.00, 40.00, 50 500 GB

What I want it be changed to is to return one product row with lowest min price and the price_diff_pct eg instead of the above return set it returns the below

1, computer table, 5.00, 10.00, 50, pvc
2, fruits basket, 15.00, 30.00, mangoes only
3, hard disk, 20.00, 40.00, 50 500 GB

And I need to order by the last_updated date.

How do I go about this?

UPDATE:

create table products   (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(300) NOT NULL,
  `last_updated` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
   PRIMARY KEY (`id`)
);
INSERT INTO products (1, 'Computer table',  '2014-07-20 09:00:00');
INSERT INTO products (2, 'Fruits basket',  '2014-07-21 09:00:00');
INSERT INTO products (3, 'Hard Disk',  '2014-07-22 09:00:00');


CREATE TABLE IF NOT EXISTS `product_prices` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `product_id` int(10) unsigned NOT NULL,
  `min_price` float(10,2) NOT NULL,
  `max_price` float(10,2) NOT NULL,
  `description` varchar(300) NOT NULL,
  PRIMARY KEY (`id`)
);
INSERT INTO product_prices (1, 1, '10.00', '20.00', 'glass top');
INSERT INTO product_prices (2, 1, '40.00', '80.00', 'hard wood');
INSERT INTO product_prices (3, 1, '5.00', '10.00', 'pvc');
INSERT INTO product_prices (4, 2, '15.00', '30.00', 'Mangoes only');
INSERT INTO product_prices (5, 3, '30.00', '60.00', '1 TB');
INSERT INTO product_prices (6, 3, '45.00', '90.00', '2 TB');
INSERT INTO product_prices (7, 3, '20.00', '40.00', '500 GB');

Answer 1

Use a sub query to get the lowest min_price for each product and join that against product price to get the rest of the details for that product pice.

Assuming that min_price is unique for a product:-

SELECT p.*, pp.*, ROUND((pp.max_price-pp.min_price)/pp.max_price*100) AS price_diff_pct 
FROM products p 
LEFT OUTER JOIN
(
    SELECT product_id, MIN(min_price) AS min_min_price
    FROM Products_Prices
    GROUP BY product_id
) sub0
ON sub0.product_id = p.id 
LEFT JOIN Product_prices pp 
ON pp.product_id = sub0.product_id
AND pp.min_price = sub0.min_min_price

EDIT

If you want to avoid a sub query you could probably do this using 2 LEFT OUTER JOINs.

This gets the products, joins that to product prices and then joins again to product prices where the 2nd product prices is less than the first, then uses the WHERE clause to exclude any where a 2nd price is found (ie, checks the 2nd price is null).

SELECT p.*, pp1.*, ROUND((pp1.max_price-pp1.min_price)/pp1.max_price*100) AS price_diff_pct 
FROM products p 
LEFT OUTER JOIN Products_Prices pp1
ON pp1.product_id = p.id 
LEFT OUTER JOIN Products_Prices pp2
ON pp2.product_id = pp1.product_id
AND pp2.min_price < pp1.min_price
WHERE pp2.product_id IS NULL

Answer 2

You can use another field (is_active ) for table Products_prices. When you insert a new row then make is_active = 'N' and you can make change in your join query accordingly.