Discount Code (20% off) but only applicable to specific products...how?

We have a discount code form field on our basket page and have worked out how to apply blanket discounts to all products however our client has asked if only certain products can be discounted based on entry of a specific discount code.

This is confusing the heck out of me because if I select "Presence of an item in cart" and then enter the product code I'll need to do that with OR statements a heap of times won't I? Actually, won't that then apply the discount regardless of what else is in the cart?

Our product are database driven btw.

Would appreciate your advice on best solution for this. Also, how about a "Discounts" and "Charges" page on your website that details the formulas for this types of things?

Thank you
NJ