Appendix A: The Xtreme.mdb ER Model

The following entity-relationship model displays the different relationships between the tables of the xtreme.mdb database. These relationships utilize primary keys, which contain the following restrictions:

• The value of the primary key must contain a unique value. In other words, no two different rows within a table can have the same value within the primary key field.

• The value of the primary key cannot contain a NULL value. A NULL value represents missing, unknown, or inapplicable data. A NULL value is not the same thing as a zero value.

Entity-relationship models are especially useful when linking multiple tables together for use within a report. Remember, when linking tables together in Crystal Reports, the linking relationships must match the relationships of the tables within the database. An entity-relationship model will tell you how different database tables link together.

The following diagram summarizes the referential integrity constraints required by the tables of the xtreme.mdb database.

Note

In the xtreme.mdb database, the Financials table does not contain any links to any other tables.

Figure A-1: The Xtreme.mdb database entity-relationship model