如何联接两个数据库表并从联接表中返回最低价格

Question

I have two tables, the first table has the product and the second table the prices. The price table could have more than one price per product but I only want to display the lowest. But I keep getting all the prices returned and I'm having trouble figuring out how to do it.

this is what I get returned in my query:

SELECT * FROM products AS pr JOIN prices AS p ON pr.id = p.product_id WHERE pr.live = 1 AND p.live = 1

id product1 name description £100
id product1 name description £300
id product1 name description £200
id product2 name description £50
id product2 name description £80
id product2 name description £60
id product3 name description £222
id product3 name description £234
id product3 name description £235

but I'm after:

id product1 name description £100
id product2 name description £50
id product3 name description £222

Any help would be appreciated

Answer 1

Group by product id an aggregate min on price

SELECT pr.*, p.m_price
FROM `products` AS pr 
  INNER JOIN (
    SELECT product_id, min(price) AS m_price
    FROM `prices`
    WHERE live = 1
    GROUP BY product_id
  ) AS p 
    ON pr.id = p.product_id 
WHERE pr.live = 1

update: something like this MIN( CAST( SUBSTR( price, 2 ) AS DECIMAL ) could help to get rid of the £ char and convert to number for correct aggregation on price

Answer 2

Judicious application of grouping and aggregate functions will achieve the desired results.

SELECT pr.id, pr.title, pr.name, pr.description, min(p.price)
  FROM products AS pr 
    JOIN prices AS p ON pr.id = p.product_id 
  WHERE pr.live = 1 AND p.live = 1
  GROUP BY pr.id

However, if the price column is a text type, it probably won't use the collation you want, and (eg) '£100' will be considered less than '£90'.