MySQL generate random product list based on budget? - mysql

Question

I'm tinkering with a function to enable users to specify a budget, and have them receive a list of randomly picked products that add up as best as possible to their specified budget.
As it stands now, I'm trying to do this with PHP but it tends to slow down when there is a huge product list.
Is this at all possible with just a MySQL query?
Edit:
My PHP "implementation":
$budget = 100;
$spent = 0;
$finished = false;
while(!$finished) {
$getRandomProduct = mysql_query('SELECT `id`, `price` FROM `products` WHERE `price` <= "'.($budget-$spent).'" ORDER BY RAND() LIMIT 1');
if(mysql_num_rows($getRandomProduct)) {
$randomProduct = mysql_fetch_assoc($getRandomProduct);
$productList[] = $randomProduct['id'];
$spent += $randomProduct['price'];
} else {
$finished = true;
}
}

Answer 1

The performance and the "randomness" of the result set can be further improved if you first define how many products to include.
$min = 10;
$max = 20;
$total = rand($min,$max);
The algorithm is based on the following:
In a collection of n positive numbers that sum up to S, at least one of them will be less than S divided by n (S/n)
Steps:
Select a product randomly where price < BUDGET /TOTAL. Get its price, lets say X.
Select a product randomly where price < (BUDGET - X)/(TOTAL - 1). Get its price, assume Y.
Select a product randomly where price < (BUDGET - X - Y)/(TOTAL - 2).
Repeat this and get (TOTAL - 1) products. For the last product, select one where price = remaining price. (or price <= remaining price and order by price desc and hopefully you could get close enough).
$budget = 100;
$spent = 0;
$finished = false;
for($i = 0; $i < $total - 1; $i++) {
$getRandomProduct = mysql_query('SELECT `id`, `price` FROM `products` WHERE `price` <= "'.(($budget-$spent)/($total - $i)).'" ORDER BY RAND() LIMIT 1');
if(mysql_num_rows($getRandomProduct)) {
$randomProduct = mysql_fetch_assoc($getRandomProduct);
$productList[] = $randomProduct['id'];
$spent += $randomProduct['price'];
} else {
break;
}
}
$getRandomProduct = mysql_query('SELECT `id`, `price` FROM `products` WHERE `price` <= "'.($budget-$spent).'" ORDER BY `price` DESC LIMIT 1');
$productList[] = $randomProduct['id'];
This improves:
The query performance, the condition is more strict
The result set is more random, as previous you could easily select the 1st product close to the budge and limit the budget of the others
References:
My answer using the same algorithm for another question