Table Items:
itemid | name
-------+--------
 1     | Teddies
 2     | Bobsleds
 3     | Pogo sticks
 4     | Trampolines

Table ShoppingCart:
cartid | name_on_cart
-------+--------------
 1     | Bob Smith
 2     | Jane Dover
 3     | Will Simpson

Table ShoppingCartItems:
cartitemid | cartid | itemid
-----------+--------+---------
 1         | 1      | 2
 2         | 3      | 4
 3         | 2      | 1
 4         | 2      | 4
Foreign key cartid -> ShoppingCart.cartid
Foreign key itemid -> Items.itemid