Figure C-6 shows the final ER model with the unique key constraints shown. Notice that for items, orders, users, and inventories, the attributes from other entities aren't included. They are instead indicated as weak entities with a double rectangle and they participate fully in the related entities as indicated by double lines.
Figure C-6. The complete ER model for the winestore database
If items, orders, and inventories are numbered across the whole system, you can omit the double rectangles. The double lines can be omitted if any entities can exist without the related entity.
A summary of ER notation tools is shown in Figure C-7.