Three level deep query in mysql using symfony2

Question

i need to find a quicker way to loop a three level deep query.

i got my customer, products and selling_date tables in a symfony2 project.

i need to generate the pricelist for each product, for each customer of each selling_date.

Normally i use to make a nested query where on each customer i loop each product on which i loop each list, so it looks like this

$c = "select * from customer"
loop each cust
  $p = "select * from product "
  loop each prod
    $l= "select * from list "
    CREATE THE PRICE LIST RECORD
  end loop
end loop

the problem is that i got 1000 customers, 20k products, 4 selling_date. as you may notice the amount of iterations is too big. so it goes on timeout, and even if i set an higher timeout, it requires a lot of time (consider that this "function" must be executed at least twice time a day)

is there a best practice to avoid this?

Thank you in advance