SQL - Closest rows to Now() but unique

Question

So I am having a slight issue. I have 2 tables, one contains the merchant information, name etc and another contains Events.

1. I upload prices to the event table every few days so there is a lot of duplicate content only distinguished through its TIMESTAMP column(Uploaddate).

2. I need to select the prices for the most recent uploads and therefore the others not to be selected.

    Event name date price merchant test 2012-02-26 £10 ShopX test 2012-02-26 £11 ShopY test 2012-02-26 £9 ShopX - LATEST PRICE 

   IE. Select prices which are the most up-to-date and make sure each merchant is unique, so only 1 price per merchant.

Here is what I have so far:

SELECT *
FROM wp_events, wp_merchants
WHERE DATE =  '2012-02-26'
AND eventname =  'testevent'
AND  `uploaddate` > NOW( ) 
AND wp_events.merchant = wp_merchants.merchant_name
ORDER BY uploaddate DESC 

Answer 1

Use MAX() in a subquery, which will look something like this:

SELECT
  MAX(DATE), event, merchant
FROM
  someTable
GROUP BY
  event, merchant

Next, incorporate the subquery into a main query to get the latest price (I guess that's what you want):

SELECT
  someTable.price, someTable.event, someTable.merchant
FROM
  someTable INNER JOIN (
    SELECT
      MAX(DATE) AS DATE, event, merchant
    FROM
      someTable
    GROUP BY
      event, merchant
  ) subquery
  ON someTable.DATE = subquery.DATE AND someTable.event = subquery.event AND someTable.merchant = subquery.merchant

I haven't verified the code.

Answer 2

Give this a try:

select e1.merchant, e1.price from event e1
left join event e2
on (e1.merchant = e2.merchant and e1.date < e2.date)
where e2.merchant is null

This will get you this result:

+----------+-------+
| MERCHANT | PRICE |
+----------+-------+
| ShopY    | £11   |
| ShopX    | £9    |
+----------+-------+

So, you'll have to add the joins and filters you need.

Edit:

Isn't this what you're looking for?

select e1.merchant, e1.price from wp_events e1
left join wp_events e2
on (e1.merchant = e2.merchant and e1.uploaddate < e2.uploaddate)
where e2.merchant is null