Figure C-6. The complete ER model for the winestore database
![figs/wda_ac06.gif](FILES/wda_ac06.gif)
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.
Figure C-7. Tools used in ER modeling
![figs/wda_ac07.gif](FILES/wda_ac07.gif)
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