Question

Problem – Retrieve sum of subtotals on a half hour interval efficiently

I am using MySQL and I have a table containing subtotals with different times. I want to retrieve the sum of these sales on a half hour interval from 7 am through 12 am. My current solution (below) works but takes 13 seconds to query about 150,000 records. I intend to have several million records in the future and my current method is too slow.

How I can make this more efficient or if possible replace the PHP component with pure SQL? Also, would it help your solution to be even more efficient if I used Unix timestamps instead of having a date and time column?

Table Name – Receipts

subtotal    date        time      sale_id
--------------------------------------------
   6        09/10/2011  07:20:33     1
   5        09/10/2011  07:28:22     2
   3        09/10/2011  07:40:00     3
   5        09/10/2011  08:05:00     4
   8        09/10/2011  08:44:00     5
...............
  10        09/10/2011  18:40:00     6
   5        09/10/2011  23:05:00     7

Desired Result

An array like this:

• Half hour 1 ::: (7:00 to 7:30) => Sum of Subtotal is 11
• Half hour 2 ::: (7:30 to 8:00) => Sum of Subtotal is 3
• Half hour 3 ::: (8:00 to 8:30) => Sum of Subtotal is 5
• Half hour 4 ::: (8:30 to 9:00) => Sum of Subtotal is 8

Current Method

The current way uses a for loop which starts at 7 am and increments 1800 seconds, equivalent to a half hour. As a result, this makes about 34 queries to the database.

for($n = strtotime("07:00:00"), $e = strtotime("23:59:59"); $n <= $e; $n += 1800) {  

    $timeA = date("H:i:s", $n);
    $timeB = date("H:i:s", $n+1799);

    $query = $mySQL-> query ("SELECT SUM(subtotal)
                              FROM Receipts WHERE time > '$timeA' 
                              AND time < '$timeB'");

    while ($row = $query-> fetch_object()) {
        $sum[] = $row;
    }
}

Current Output

Output is just an array where:

• [0] represents 7 am to 7:30 am
• [1] represents 7:30 am to 8:00 am

• [33] represents 11:30 pm to 11:59:59 pm.

  array (“0” => 10000,
  “1” => 20000,
  …………..
  “33” => 5000);

Answer 1

You can try this single query as well, it should return a result set with the totals in 30 minute groupings:

SELECT date, MIN(time) as time, SUM(subtotal) as total
FROM `Receipts`
WHERE `date` = '2012-07-30'
GROUP BY hour(time), floor(minute(time)/30)

To run this efficiently, add a composite index on the date and time columns.

You should get back a result set like:

+---------------------+--------------------+
| time                | total              |
+---------------------+--------------------+
| 2012-07-30 00:00:00 |        0.000000000 |
| 2012-07-30 00:30:00 |        0.000000000 |
| 2012-07-30 01:00:00 |        0.000000000 |
| 2012-07-30 01:30:00 |        0.000000000 |
| 2012-07-30 02:00:00 |        0.000000000 |
| 2012-07-30 02:30:00 |        0.000000000 |
| 2012-07-30 03:00:00 |        0.000000000 |
| 2012-07-30 03:30:00 |        0.000000000 |
| 2012-07-30 04:00:00 |        0.000000000 |
| 2012-07-30 04:30:00 |        0.000000000 |
| 2012-07-30 05:00:00 |        0.000000000 |
| ...
+---------------------+--------------------+