mysql使用MAX（date）获取最后N条记录

Question

So I have following data in a product_rate_history table -

I want to select last N records ( eg 7 records ) informing rate change history of given product. If product rate is changed more than one time a day, then query should select most recent rate change for that day.

So from above table I want output like following for product id 16 -

+-----------+-------------------------+------------------------+
|  product_id    |    previous_rate   |           date         |
+----------------+--------------------+------------------------|
|      16        |      2400          |   2016-04-30 23:05:35  |
|      16        |      4500          |   2016-04-29 11:02:42  |
+----------------+--------------------+------------------------+

I have tried following query but it returns only one row having last update rate only-

SELECT * FROM `product_rate_history` prh
    INNER JOIN (SELECT max(created_on)  as max FROM `product_rate_history` GROUP BY Date(created_on)) prh2
      ON prh.created_on = prh2.max
    WHERE prh.product_id = 16
    GROUP BY DATE(prh.created_on)
    ORDER BY prh.created_on DESC;

Answer 1

First, you do not need an aggregation in the outer query.

Second, you need to repeat the WHERE clause in the subquery (for the method you are using):

SELECT prh.*
FROM product_rate_history prh INNER JOIN
     (SELECT max(created_on) as maxco
      FROM product_rate_history
      WHERE prh.product_id = 16
      GROUP BY Date(created_on)
     ) prh2
     ON prh.created_on = prh2.maxco
WHERE prh.product_id = 16
ORDER BY prh.created_on DESC;