Business Rules - Database Design for Mere Mortals - page 338

The response in Step 2 suggests two possible constraints that you should impose upon the
C UST C OUNTY field: A county name is required for each customer, and the range of values
forthis field is limited tofourspecific counties (the two currently onthe field specification
and the two new counties indicated in the response). Here are two statements you might
use to begin transforming these constraints into business rules:
A county must be associated with each customer.
The only counties that can be entered into this field are King, Kit-
sap, Pierce, and Snohomish.
Once you've defined the appropriate business rules, you can move on to Step 4.
Step 4: Establish the Rules by Modifying the Appropriate Field Specification Elements
Establish each business rule you defined in Step 3 by modifying the appropriate elements
on the Field Specifications sheet. (Remember that some rules may affect more than one
element.) First, however, you must identify which elements of the field specifications the
rule affects. For example, consider the first business rule you defined for the CustCounty
field in Step 3:
A county must be associated with each customer.
You can deduce that the rule affects the Required Value, Null Support, and Edit Rule ele-
ments because it explicitly states that a county “must be associated” with a customer. Now
youcanmaketheappropriatemodificationstotheseelements.Inthisparticularcase,you'll
set Required Value to “Yes,” Null Support to “No Nulls,” and Edit Rule to “Enter Now,
Edits Allowed.”
Asyoucansee,it'simportant foryoutoexamine eachbusinessruleverycarefully inorder
to determine which field specification elements it's going to affect. When you first begin
to define business rules, it's best to have a Field Specifications sheet handy so that you can
refertoitasnecessary.Manyoftheelementswillcometomindmoreeasilyasyoubecome
more experienced at establishing business rules.
Now, consider the next business rule in the example:
The only counties that can be entered into this field are King, Kit-
sap, Pierce, and Snohomish.
This business rule affects the Range of Values element, and you'll now revise its setting to
“King, Kitsap, Pierce, and Snohomish.” Figure 11.7 shows the revised Logical Elements
category of the Field Specifications sheet for the C UST C OUNTY field.