ENABLE User's Guide

DEVELOPING A DATA BASE
Choosing Key Fields
A key-sequenced file type is the best choice for both files in
the personnel data base. A relative file type is not necessary
because no logical use for the record number field exists. An
entry-sequenced file type is not satisfactory because the
personnel department enters data in both files randomly and
sequentially. An unstructured file type is unsatisfactory
because, with such a file type, the personnel department could
not use alternate keys to find an "employee" or "depend" record.
After you select a file type for a file, you are ready to choose
key fields for that file.
Choosing Key Fields
Assuming that you have selected a key-sequenced file type for
your data base files, you must now choose one primary key field
for each. In addition, you can choose up to 255 alternate key
fields for each file.
For the "employee" file, a good choice for a primary key is
"empnum." One of the requirements for a primary key field is
that it stores unique values. Since each employee has a unique
employee number, "empnum" satisfies this requirement. Good
candidates for alternate keys in this file are "empname,"
"regnum," and "branchnum." Instead of making both "regnum" and
"branchnum" separate alternate key fields, you could identify a
group field ("dept") of which these fields are a part and make
that group an alternate key field. When you choose a group as a
key field, that group is said to be a composite key field.
For the "depend" file, neither "emp-no" nor "dependent-name"
uniquely identify a record. These fields are not unique because
the same employee number can appear with several dependents, and
several dependents could have the same name. If you provide a
number that identifies each dependent of a given employee, you
could identify a group ("dep-key") consisting of "emp-no" and the
dependent number ("dependent-no"). You could then use this group
as a composite primary-key field. Although "dependent-name" is
not unique, and therefore cannot serve as a primary key, it is a
good choice for an alternate key field.
When you are choosing key fields for your files, consider making
any field (such as "empnum") that also appears in another file a
key field of the other file. If you do this, you can use ENABLE
to generate an application that can access both files.
3-10