Relational Tables

Now suppose that you had multiple catalog items created by the same vendor. Where would you store the vendor information (things like vendor name, address, and contact information)? You wouldn't want to store that data along with the products for several reasons:

• Because the vendor information is the same for each product that vendor produces, repeating the information for each product is a waste of time and storage space.

• If vendor information changes (for example, if the vendor moves or his area code changes), you would need to update every occurrence of the vendor information.

• When data is repeated (that is, the vendor information is used with each product), there is a high likelihood that the data will not be entered exactly the same way each time. Inconsistent data is extremely difficult to use in reporting.

The key here is that having multiple occurrences of the same data is never a good thing, and that principle is the basis for relational database design. Relational tables are designed so that information is split into multiple tables, one for each data type. The tables are related to each other through common values (and thus the relational in relational design).

In our example, you can create two tables, one for vendor information and one for product information. The Vendors

转载于:https://www.cnblogs.com/alexusli/archive/2009/03/27/1423009.html