Question

For a personal project I’m working on right now I want to make a line graph of game prices on Steam, Impulse, EA Origins, and several other sites over time. At the moment I’ve modified a script used by SteamCalculator.com to record the current price (sale price if applicable) for every game in every country code possible or each of these sites. I also have a column for the date in which the price was stored. My current tables look something like so:

THIS STRUCTURE IS NO LONGER VALID. SEE BELOW
+----------+------+------+------+------+------+------+------------+
| steam_id |  us  |  at  |  au  |  de  |  no  |  uk  |    date    |
+----------+------+------+------+------+------+------+------------+
|  112233  |  999 |  899 |  999 | NULL |  899 |  699 |  2011-8-21 |
|  123456  | 1999 |  999 | 1999 |  999 |  999 |  999 |  2011-8-20 |
|    ...   |  ... |  ... |  ... |  ... |  ... |  ... |     ...    |
+----------+------+------+------+------+------+------+------------+

At the moment each country is updated separately (there’s a for loop going through the countries), although if it would simplify it then this could be modified to temporarily store new prices to an array then update an entire row at a time. I’ll likely be doing this eventually, anyway, for performance reasons.

Now my issue is determining how to best update this table if one of the prices changes. For instance, let’s suppose that on 8/22/2011 the game 112233 goes on sale in America for $4.99, Austria for 3.99€, and the other prices remain the same. I would need the table to look like so:

THIS STRUCTURE IS NO LONGER VALID. SEE BELOW
+----------+------+------+------+------+------+------+------------+
| steam_id |  us  |  at  |  au  |  de  |  no  |  uk  |    date    |
+----------+------+------+------+------+------+------+------------+
|  112233  |  999 |  899 |  999 | NULL |  899 |  699 |  2011-8-21 |
|  123456  | 1999 |  999 | 1999 |  999 |  999 |  999 |  2011-8-20 |
|    ...   |  ... |  ... |  ... |  ... |  ... |  ... |     ...    |
|  112233  |  499 |  399 |  999 | NULL |  899 |  699 |  2011-8-22 |
+----------+------+------+------+------+------+------+------------+

I don’t want to create a new row EVERY time the price is checked, otherwise I’ll end up having millions of rows of repeated prices day after day. I also don’t want to create a new row per changed price like so:

THIS STRUCTURE IS NO LONGER VALID. SEE BELOW
+----------+------+------+------+------+------+------+------------+
| steam_id |  us  |  at  |  au  |  de  |  no  |  uk  |    date    |
+----------+------+------+------+------+------+------+------------+
|  112233  |  999 |  899 |  999 | NULL |  899 |  699 |  2011-8-21 |
|  123456  | 1999 |  999 | 1999 |  999 |  999 |  999 |  2011-8-20 |
|    ...   |  ... |  ... |  ... |  ... |  ... |  ... |     ...    |
|  112233  |  499 |  899 |  999 | NULL |  899 |  699 |  2011-8-22 |
|  112233  |  499 |  399 |  999 | NULL |  899 |  699 |  2011-8-22 |
+----------+------+------+------+------+------+------+------------+

I can prevent the first problem but not the second by making each (steam_id, <country>) a unique index then adding ON DUPLICATE KEY UPDATE to every database query. This will only add a row if the price is different, however it will add a new row for each country which changes. It also does not allow the same price for a single game for two different days (for instance, suppose game 112233 goes off sale later and returns to $9.99) so this is clearly an awful option.

I can prevent the second problem but not the first by making (steam_id, date) a unique index then adding ON DUPLICATE KEY UPDATE to every query. Every single day when the script is run the date has changed, so it will create a new row. This method ends up with hundreds of lines of the same prices from day to day.

How can I tell MySQL to create a new row if (and only if) any of the prices has changed since the latest date?

UPDATE –

At the recommendation of people in this thread I have changed the schema of my database to facilitate adding new country codes in the future and avoid the issue of needing to update entire rows at a time. The new schema looks something like:

+----------+------+---------+------------+
| steam_id |  cc  |  price  |    date    |
+----------+------+---------+------------+
|  112233  |  us  |   999   |  2011-8-21 |
|  123456  |  uk  |   699   |  2011-8-20 |
|    ...   |  ... |   ...   |     ...    |
+----------+------+---------+------------+

On top of this new schema I have discovered that I can use the following SQL query to grab the price from the most recent update:

SELECT `price` FROM `steam_prices` WHERE `steam_id` = 112233 AND `cc`='us' ORDER BY `date` ASC LIMIT 1

At this point my question boils down to this:

Is it possible to (using only SQL rather than application logic) insert a row only if a condition is true? For instance:

INSERT INTO `steam_prices` (...) VALUES (...) IF price<>(SELECT `price` FROM `steam_prices` WHERE `steam_id` = 112233 AND `cc`='us' ORDER BY `date` ASC LIMIT 1)

From the MySQL manual I can not find any way to do this. I have only found that you can ignore or update if a unique index is the same. However if I made the price a unique index (allowing me to update the date if it was the same) then I would not be able to recognize when a game went on sale and then returned to its original price. For instance:

+----------+------+---------+------------+
| steam_id |  cc  |  price  |    date    |
+----------+------+---------+------------+
|  112233  |  us  |   999   |  2011-8-20 |
|  112233  |  us  |   499   |  2011-8-21 |
|  112233  |  us  |   999   |  2011-8-22 |
|    ...   |  ... |   ...   |     ...    |
+----------+------+---------+------------+

Also, after just finding and reading MySQL Conditional INSERT, I created and tried the following query:

INSERT INTO `steam_prices`(
    `steam_id`,
    `cc`,
    `update`,
    `price`
)
SELECT '7870', 'us', NOW(), 999
FROM `steam_prices`
WHERE
    `price`<>999
    AND `update` IN (
        SELECT `update`
        FROM `steam_prices`
        ORDER BY `update`
        ASC LIMIT 1
    )

The idea was to insert the row '7870', 'us', NOW(), 999 if (and only if) the price of the most recent update wasn’t 999. When I ran this I got the following error:

1235 – This version of MySQL doesn’t yet support ‘LIMIT & IN/ALL/ANY/SOME subquery’

Any ideas?

Answer 1

After experimentation, and with some help from MySQL Conditional INSERT and http://www.artfulsoftware.com/infotree/queries.php#101, I found a query that worked:

INSERT INTO `steam_prices`( 
    `steam_id`, 
    `cc`, 
    `price`,
    `update` 
) 
SELECT 7870, 'us', 999, NOW() 
FROM `steam_prices` AS p1
LEFT JOIN `steam_prices` AS p2 ON p1.`steam_id`=p2.`steam_id` AND p1.`update` < p2.`update`
WHERE 
    p2.`steam_id` IS NULL
    AND p1.`steam_id`=7870
    AND p1.`cc`='us'
    AND (
        p1.`price`<>999
    )

The answer is to first return all rows where there is no earlier timestamp. This is done with a within-group aggregate. You join a table with itself only on rows where the timestamp is earlier. If it fails to join (the timestamp was not earlier) then you know that row contains the latest timestamp. These rows will have a NULL id in the joined table (failed to join).

After you have selected all rows with the latest timestamp, grab only those rows where the steam_id is the steam_id you’re looking for and where the price is different from the new price that you’re entering. If there are no rows with a different price for that game at this point then the price has not changed since the last update, so an empty set is returned. When an empty set is returned the SELECT statement fails and nothing is inserted. If the SELECT statement succeeds (a different price was found) then it returns the row 7870, 'us', 999, NOW() which is inserted into our table.

EDIT – I actually found a mistake with the above query a little while later and I have since revised it. The query above will insert a new row if the price has changed since the last update, but it will not insert a row if there are currently no prices in the database for that item.

To resolve this I had to take advantage of the DUAL table (which always contains one row), then use an OR in the where clause to test for a different price OR an empty set

INSERT INTO `steam_prices`( 
    `steam_id`, 
    `cc`, 
    `price`,
    `update` 
) 
SELECT 12345, 'us', 999, NOW() 
FROM DUAL
WHERE
    NOT EXISTS (
        SELECT `steam_id`
        FROM `steam_prices`
        WHERE `steam_id`=12345
    )
    OR
    EXISTS (
        SELECT p1.`steam_id`
        FROM `steam_prices` AS p1 
        LEFT JOIN `steam_prices` AS p2 ON p1.`steam_id`=p2.`steam_id` AND p1.`update` < p2.`update`
        WHERE 
            p2.`steam_id` IS NULL 
            AND p1.`steam_id`=12345 
            AND p1.`cc`='us' 
            AND ( 
                p1.`price`<>999
            )
    )

It’s very long, it’s very ugly, and it’s very complicated. But it works exactly as advertised. If there is no price in the database for a certain steam_id then it inserts a new row. If there is already a price then it checks the price with the most recent update and, if different, inserts a new row.