Suppose you run a Web site that includes a banner
advertisement service. You contract with companies
that want their ads displayed when poeple visit
the pages on your site. Each time a visitor hits
one of your pages, you serve an ad embedded in the
page that is sent to the visitor's browser and
assess the company a small fee. To represent this
information, you maintain three tables. One table,
company, has columns for company name, number,
address, and telephone number. Another table, ad
lists ad numbers, the number for the company that
owns the ad, and the amount you charge per hit.
The third table, hit, logs each ad hit by number
and the date on which the ad was
served.