Using MYSQL, SELECT every nth row from a subquery SELECT - php

Question

I have a table of over 300,000 rows and I would like to render this data on a graph, but 300,000 rows isn't really necessary all at once. For example, even though there may be 100 rows of data for a given day, I don't need to display all that data if I'm showing a whole year worth of data. So I would like to "granularize" the data.
I was thinking of getting everything and then using a script to remove what I don't need, but that seems like it would be much slower and harder on the database.
So here's what I have so far.
SET #row_number := 0;
SELECT #row_number := #row_number + 1 as row_number,
price, region, timestamp as row_number FROM pricehistory;
This gives me all the rows and numbers them. I was planning on adding a where clause to get every 1000 rows (i.e. every nth row) like this
SET #row_number := 0;
SELECT #row_number := #row_number + 1 as row_number,
price, region, timestamp as row_number FROM pricehistory
WHERE row_number % 1000 = 0;
But MYSQL doesn't see row_number as a column for some reason. Any ideas? I've looked at other solutions online, but they don't seem to work for MYSQL in particular.

Answer 1

As Racil's comment suggested, you can just go by an auto-incremented id field if you have one; but you've stated the amount of data for different dates could be different, so this could make for a very distorted graph. If you select every 1000th record for a year and half the rows are from the last 3 months ("holiday shopping" for a commerce example), the latter half of a year graph will actually reflect the latter quarter of the year. For more useful results you're most likely better off with something like this:
SELECT region, DATE(timestamp) AS theDate
, AVG(price), MIN(price), MAX(price)
FROM pricehistory
GROUP BY region, theDate
;

Answer 2

It doesn't look like I'm going to get another answer so I'll go ahead and write the solution I came up with.
My data is pretty evenly distributed as it grabs prices at regular intervals so there's no reason to worry about that.
Here's my solution.
Let's say I have 500,000 rows and I want to display a subset of those rows let's say 5000 rows. 500000/5000 is 100 so I take 100 and use it in my select statement like this SELECT * FROM pricehistory where id % 100 = 0;
Here is the actual code
public function getScaleFactor($startDate, $endDate) {
$numPricePoints = $this->getNumPricePointsBetweenDates($startDate, $endDate);
$scaleFactor = 1;
if ($numPricePoints > $this->desiredNumPricePoints) {
$scaleFactor = floor($numPricePoints / $this->desiredNumPricePoints);
}
return $scaleFactor;
}
I then use $scaleFactor in the SQL like this SELECT * FROM pricehistory WHERE id % {$scaleFactor} = 0;
This isn't a perfect solution because you don't always end up with 5000 rows exactly, but I don't NEED exactly 5000 rows. I'm just trying to reduce the resolution of the data while still getting a graph that looks close to what it would be had I used all 500,000 rows.