Field Specifications - Database Design for Mere Mortals - page 243

Figure 9.8. Unique field specification for the V ENDOR ID N UMBER field
Here are a few things to note about this specification.
1. This field also appears in the PRODUCTS table, as indicated by the Shared By
general element. This is both reasonable and necessary because each product must
be associated with a specific vendor. (You'll learn more about this type of issue in
the next chapter.)
2. Examine the settings for the Uniqueness, Null Support, Required Value, and Edit
Rule logical elements. They are set in this manner because the Key Type element
is set to “Primary.” You should, in fact, use these element settings for any field that
serves as a primary key.
3. The Comparisons Allowed logical element is set to “Same Field—Equals” so that
a user can compare V ENDOR ID N UMBER values in the VENDORS table to V ENDOR
ID N UMBER values in the PRODUCTS table.
4. The Comparisons Allowed logical element is also set to “Value Expres-
sion—Equals” so that a user can compare V ENDOR ID N UMBER values to some ar-
bitrary numeric value.
Figure 9.9 shows the complete Generic field specification for a generic STATE field.