SQL to find the date when the price last changed - sql

Question

Input:
Date Price
12/27 5
12/21 5
12/20 4
12/19 4
12/15 5
Required Output:
The earliest date when the price was set in comparison to the current price.
For e.g., price has been 5 since 12/21.
The answer cannot be 12/15 as we are interested in finding the earliest date where the price was the same as the current price without changing in value(on 12/20, the price has been changed to 4)

Answer 1

This should be about right. You didn't provide table structures or names, so...
DECLARE #CurrentPrice MONEY
SELECT TOP 1 #CurrentPrice=Price FROM Table ORDER BY Date DESC
SELECT MIN(Date) FROM Table WHERE Price=#CurrentPrice AND Date>(
SELECT MAX(Date) FROM Table WHERE Price<>#CurrentPrice
)
In one query:
SELECT MIN(Date)
FROM Table
WHERE Date >
( SELECT MAX(Date)
FROM Table
WHERE Price <>
( SELECT TOP 1 Price
FROM Table
ORDER BY Date DESC
)
)

Answer 2

This question kind of makes no sense so im not 100% sure what you are after.
create four columns, old_price, new_price, old_date, new_date.
! if old_price === new_price, simply print the old_date.

Answer 3

What database server are you using? If it was Oracle, I would use their windowing function. Anyway, here is a quick version that works in mysql:
Here is the sample data:
+------------+------------+---------------+
| date | product_id | price_on_date |
+------------+------------+---------------+
| 2011-01-01 | 1 | 5 |
| 2011-01-03 | 1 | 4 |
| 2011-01-05 | 1 | 6 |
+------------+------------+---------------+
Here is the query (it only works if you have 1 product - will have to add a "and product_id = ..." condition on the where clause if otherwise).
SELECT p.date as last_price_change_date
FROM test.prices p
left join test.prices p2 on p.product_id = p2.product_id and p.date < p2.date
where p.price_on_date - p2.price_on_date <> 0
order by p.date desc
limit 1
In this case, it will return "2011-01-03".
Not a perfect solution, but I believe it works. Have not tested on a larger dataset, though.
Make sure to create indexes on date and product_id, as it will otherwise bring your database server to its knees and beg for mercy.
Bernardo.