Organizing member join dates/member growth data into graphable chunks - php

Question

I'm dabbling with pChart and would like to start with a simple line graph showing the growth in membership over time.
Y-axis would be # of membersX-axis would be time
For each time datapoint, I need a corresponding total members datapoint.
My user table is structured as:
[user_id] [join_date]
The approach I came up with on the bus to work this morning is:
$Q = " SELECT MONTH(join_date), DAY(join_date), COUNT(user_id)"
. " FROM user_basic_data GROUP BY join_date";
$R = mysql_query($Q);
$dateS = '';
$totalS = '';
$c = 0; // total members counter
while ($row = mysql_fetch_row($R)) {
$dateS .= $row[0].'-'.$row[1].','; // month-day,month-day,month-day
$c = $row[2] + $c; // new total for new date
$totalS .= $c.','; // total1,total2,total3
}
// trim trailing commas
$dateS = substr($dateS, 0, -1);
$totalS = substr($totalS, 0, -1);
echo "<p>$dateS</p>"; // Ex: 8-10,8-15,8-20
echo "<p>$totalS</p>"; // Ex: 12,17,23
Those string formats are how pChart likes the data, and I know the current query would need a year value as well for real use, so please don't get hung up on those points.
I'd like to know if there's a better way to go about getting the changing total members over time. I'm guessing handling it within MySQL would be faster, but I can't think of a way to do that.
Thank you for your time.

Answer 1

To get a running total, use:
SELECT DISTINCT
DATE(ubd.join_date) AS dt,
(SELECT COUNT(*)
FROM user_basic_data t
WHERE DATE(t.join_date) <= DATE(ubd.join_date)) AS num_users
FROM user_basic_data ubd
DATE returns dates as YYYY-MM-DD; If you still want Month-Day - use DATE_FORMAT by replacing the DATE(udb.join_date) with:
DATE_FORMAT(ubd.join_date), '%m-%d')
You don't need the logic to create the comma separated lists in PHP - just need to populate the two variables:
$Q = " SELECT GROUP_CONCAT(x.dt) AS dates,
GROUP_CONCAT(x.num_users) AS totals
FROM (SELECT DISTINCT
DATE(ubd.join_date) AS dt,
(SELECT COUNT(*)
FROM user_basic_data t
WHERE DATE(t.join_date) <= DATE(ubd.join_date)) AS num_users
FROM user_basic_data ubd ) x";
$R = mysql_query($Q);
while ($row = mysql_fetch_row($R)) {
echo "<p>$row[0]</p>"; // Ex: 8-10,8-15,8-20
echo "<p>$row[1]</p>"; // Ex: 12,17,23
}