sc_cart will be only a single entry and it just stores the owner name of the cart ie the requester. Every time you add an item to cart, a new entry is added in the sc_cart_item. Once you order or submit the cart, the entry added in the cart is removed from the sc_cart_item table and is converted to a request and sc_req_item entries. So sc_cart always remains the same but requests get added or removed from the sc_cart_item table.