Field Specifications - Database Design for Mere Mortals - page 226

type of specification, and the element settings you establish will apply only to the
field indicated in the Field Name element.
2. Generic: This specification serves as a template for other field specifications and
helps you ensure consistent definitions for fields that have the same general mean-
ing. For example, you could create this type of specification for a generic S TATE
field and then use it as the basis for every other S TATE field in the database. Fields
such as C UST S TATE , E MP S TATE , and V END S TATE all have the same meaning (they
represent a state within the United States), but there is enough of an obvious dis-
tinction between them to require that they remain separate fields. (If you recall,
you learned about generic fields in Chapter 6 , “ Analyzing the Current Database ,”
when you were developing the Preliminary Field List and in Chapter 7 when you
were working with the Elements of the Ideal Field.)
A generic specification requires you to use a nonspecific field name and element
settings that are as broad and general as possible. You can, however, incorporate
any element except Parent Table, Label, Shared By, Alias(es), and Source Specific-
ation.
3. Replica: This is the default specification for a field based on a generic field or a
field that serves as a foreign key within a table relationship, and it draws a majority
of its element settings from an existing specification. You can incorporate elements
that were not already incorporated by the source specification, and you can alter
any element settings drawn from the source specification.
You'lllearnhowtodefineeachtypeofspecificationinthesection“ UsingUnique,Generic,
and Replica Field Specifications ” later in this chapter.
Source Specification
This element is set only on a Replica specification and indicates the name of the specific
field specification upon which the current specification is based. (You'll see a good ex-
ample of this element in the next section.)
Shared By
This element indicates the names of other tables that share this field. The only table names
that should appear here are those that have an explicit relationship to the field's parent
table. For example, assume you have a data table called EMPLOYEES that is related to
two subset tables called PART-TIME EMPLOYEES and FULL-TIME EMPLOYEES via
a field called E MPLOYEE ID N UMBER . As you create a field specification for E MPLOYEE ID
N UMBER ,youwoulduse“PART-TIMEEMPLOYEES,FULL-TIMEEMPLOYEES”asthe
setting for this element.