Question

I have a table called “items”

id       month       year       **itemname**       distmoney
 1        12         2012         chicken           20
 2        12         2012         pork              15
 3        11         2012         chicken           21
 4        11         2012         pork              15

I am trying to find the difference between the “distmoney” for the same itemname between two months.

Example:

ID 1, itemname chicken. month 12 distmoney is 20, while month 11 distmoney is 21. I want to be able to calculate the difference of 1 for id=1, itemname=chicken

Right now I have the php code to calculate the difference between two numbers, but I am having a tough time figuring out how to grab the previous month distmoney.

<?php foreach($rows as $row): ?>
<?php $number1 = htmlentities($row['distmoney']) ?>
<?php endforeach; ?>
<?php
$number1 = $row['distmoney'];
$number2 = ????????; // THIS NEEDS TO BE THE PREVIOUS MONTH DISTMONEY VALUE
if ($number1 <= $number2) {
$difference = "(Price Lowered) Price difference of $";
$result = $number2 - $number1;
$percent = round(100.0*($number2-$number1)/$number1);
echo $difference; echo $result; echo $percent; echo "%";
} elseif ($number1 > $number2) {
$result = $number1 - $number2;
$percent = round(100.0*($number2/$number1-1));
$addition = "(Price Higher) Price difference of $";
echo $addition; echo $result; echo $percent; echo "%";
} 

?>

Answer 1

If you want to perform this in SQL, then you can transform the data in columns to then get the difference:

select 
  itemname,
  max(case when month = 12 and year = 2012 then distmoney else 0 end) Dec2012,
  max(case when month = 11 and year = 2012 then distmoney else 0 end) Nov2012
from yourtable
group by itemname

See SQL Fiddle with Demo

The result of the query is:

| ITEMNAME | DEC2012 | NOV2012 |
--------------------------------
|  chicken |      20 |      21 |
|     pork |      15 |      15 |