在多个表中使用foreach

So, I have a database table called customers. The last 6 products they have purchased is stored in the customer table under "p1, p2, p3, p4, p5 and p6". The product would then be stored as the product number, eg

Last Purchased:
   p1: 12355
   p2: 55142
   p3: 51251

etc. These products all have they're own table row in the "Products" table. On that list is amongst others "price". Now, what I would like to do is an somewhat easy way to extract the biggest purchase amongst the last 6 purchases. Getting the biggest is easy by using the max() command, and I could do something like the whole

$p1 = $array[0]["p1"];
$array = $db->queryA("SELECT * FROM products WHERE product = $p1");

and then compare them as in max($p1,$p2,$p3) etc. My problem is this would take an enormous amount of time and code. Is there a better way of doing this, for instance with an foreach loop? I am really sorry, but I am completely stomped. Again.. All help is appreciated. :)

Edit: not sure how to display the table structure, but here goes my attempt, sorry if it is bad :(

customers            products
   id                      prod_nr     
   name                    name
   mail                    price
   member_since            quantity
   p1
   p2
   p3
   p4
   p5
   p6

Would this suffice? It is some more fields, but it is the gist of it. Also, "id" and "prod_nr" are primaries

edit nr2: also wanted to explain that I have about 11 of these 6* queries I would like to run, so as to not making you think I'm just wasting your time instead of a little of mine... :)

Edit again, here is the part of the database provider giving me problems...

    public function query($query)
{
    try{
        $query = trim($query);
        $sth = $this->pdo->query($query,PDO::FETCH_ASSOC);
        $result = null;
        foreach($sth as $row) {
            $result[] = $row;
        }
        return $result != null ? json_encode($result) : null;
    }
    catch(exception $e)
    {
        echo('Exception ' . $e);
    }
}

What you could do is all the items to an array, and then implode it so it uses single quotes and then execute a query like this:

$array = $db->queryA("SELECT * FROM products WHERE product IN($your_array)");

First you need is redesign your database you need a purchase table like this

purchase
---------
id_purchase
id_customer
id_product

customers            
-----------
id                           
name                    
mail                    
member_since            

products
------------
prod_nr 
name
price
quantity

Then your SQL would be:

SELECT * from customers JOIN purchase ON id = id_customer JOIN products ON prod_nr = id_product
ORDER BY DESC price
LIMIT 1