IF ELSE statement within Select query

[Sahand S.]

I tried researching this topic but I am new to MySQL and was unable to find answers, I could understand.

I need to pull some product information and also calculate the subtotal by multiplying the number of units selected by the price.
My problem is that I want to add a volume price for some items. So if volqty is 5 and the visitor wants more than 5 items they would be charged a lower price, else just use the normal price.

After a quick search I found some examples of CASE and here is what I wrote:
(Could anyone tell me if this is ok and/or if there is a simpler way to do the same.)

$query_string = "SELECT products.product_id,
products.price,
products.unit,
products.volqty,
products.volprice,
shopping_cart.quantity,
CASE WHEN products.volqty > 0 AND products.unit >= products.volqty THEN products.volprice ELSE products.price END * shopping_cart.quantity AS subtotal
FROM shopping_cart INNER JOIN products
ON shopping_cart.product_id = products.product_id
WHERE shopping_cart.cart_id = '$cartId'";

THANK YOU in advance, and I am sorry if this has already been covered.