Find difference between two values for each row where "number" column value is same

Question

month   year    customer    distributor    number   name    price   
10      2012    20          8              2406163 CHEESE   50.4
10      2012    20          8              2325141 APPLE    25.2
11      2012    20          8              2406163 CHEESE   48.1
11      2012    20          8              2325141 APPLE    20.2
12      2012    20          8              2325141 APPLE    23.2
12      2012    20          8              2406163 CHEESE   46.4

I am attempting to compare two months where "number" is the same value, I'd like to find the difference between the price (if any).

Using the data above, I would want to compare months 11 & 12 (or 10 and 11, depending of these are chosen by the user). So, for months 11&12, price difference for cheese is -$1.7 and apple is -$3. This is what I am trying to do, except it should be doing this for ALL rows found in the table between two months where "number" is the same.

Here is my current code...it is currently comparing by item name rather than item number. However, for whatever reason it only finds the difference for 3 items out of 200plus items, strangely.

Thanks for any assistance.

<?PHP
$from_date = $from_month;
$to_date = $to_month;
$query = " 
   select * from ogi
   where month BETWEEN '" . $from_date . "' AND  '" . $to_date . "' GROUP BY number HAVING count(*) > 1 ;
"; 
try 
{ 
    // These two statements run the query against your database table. 
    $stmt = $db->prepare($query); 
    $stmt->execute(); 
} 
catch(PDOException $ex) 
{ 
    // Note: On a production website, you should not output $ex->getMessage(). 
    // It may provide an attacker with helpful information about your code.  
    die("Failed to run query: " . $ex->getMessage()); 
} 

// Finally, we can retrieve all of the found rows into an array using fetchAll 
$rows = $stmt->fetchAll(); 

//Create a variable to hold the "previous month" values
$previousname = '';
$previousprice = '';

//Check each record from your query
foreach($rows as $row) {

//If not first time around && same item name as previous
if($previousname != '' && $previousname == $row['name']) {

  //subtraction calculation here

  if ($row['price'] <= $previousprice) {
$difference = "(Price Lowered) Price difference of $";
$result = $previousprice - $row['price'];
$percent = round(100.0*($previousprice/$row['price']-1));

   ?>
            <tr>
                <td><?php echo "" . $row['name'] . ""?></td>
                <td><?php echo $difference; ?><?php echo $result ?>        </td>
                <td><?php echo $percent; ?> %</td>

            </tr>
            <?
} elseif ($row['price'] > $previousprice) {
$result = $row['price'] - $previousprice;
$percent = round(100.0*($previousprice/$row['price']-1));
$addition = "(Price Higher) Price difference of $";

 ?>
            <tr>
                <td><?php echo "" . $row['name'] . ""?></td>
                <td><?php echo $addition; ?><?php echo $result ?>    </td>
                <td><?php echo $percent; ?>%</td>

            </tr>
            <?
} 
   }
   else {
   ?>
   <!-- <tr>
                <td><?php echo "" . $row['name'] . ""?></td>
                <td></td>
                <td></td>

            </tr> -->
            <?

}

//Assign the "previous month" value
$previousname = $row['name'];
$previousprice = $row['price'];

}


?>
<?
}
else {
?>
<form action="" method="post">
Choose Months to Compare:<br>
Customer: <input type="text" name="customer"><br>
Month 1: <input type="text" name="from_month"><br>
Month 2: <input type="text" name="to_month"><br>
<input type="submit" value="Compare">
</form>
<?
}
?>

Answer 1

What about something like:

SELECT a.number, a.name, (a.price - b.price) as pdiff
FROM table a, table b
WHERE a.number = b.number 
    AND a.month = montha 
    AND b.month = monthb;

If I didn't make any mistake this query should give you the needed info:

(where montha = 10 and monthb = 11 for example)

Hope this helps.