Database Reference
In-Depth Information
In this example, the E
MPLOYEE
ID N
UMBER
field identifies the person who manages a par-
ticular department. Assuming that a person is allowed to manage only one department at
any given time, the values in this field should be unique; therefore, you should set the
Uniqueness element for this field as “Unique.”
Null Support
This specifies whether a field accepts null values. “No Nulls” is the setting you'll com-
monly use for this element, especially when a field serves as a primary key or an alternate
key, or when the field's Required Value element is set to “Yes.” You can set this element
to “Nulls Allowed,” however, when there is a valid reason for a field to accept null values.
A C
UST
C
OUNTY
field, for example, must accept nulls because a customer may not know
the name of the county in which she lives. (Of course, it will no longer be null once she
supplies the county name.)
Rememberthatanulldoesnotrepresentablank—itrepresentsa
missing
or
unknown
value.
Users commonly make the mistake of using a blank to represent a
meaningful
value, such
as “None,” “Not Applicable,” “No Response,” and “Not Wanted.” If these values are valid
for a particular field, then make sure you include them in the Range of Values element for
the field. Above all, use nulls judiciously and
do not
use blanks!
Values Entered By
This element indicates the source of a field's values. Either a user will enter values into
the field manually or a database application program will enter them automatically; the ap-
plication program can provide values for the field only if the person who developed the
programprovidedameansforittogeneratethevalues.Notethatthesettingthatrepresents
the database application program is “System.”
Required Value
Thisdenoteswhetherauserisrequiredtoenteravalueforafield.Althoughyou'lltypically
set this element to “No” for most of the fields in a table, you
must
set it to “Yes” when the
field serves as the primary key. You may also need to set Required Value to “Yes” for a
field such as C
UST
Z
IPCODE
—a letter or package you send to a given customer must include
a zip code in order for the Postal Service to handle it properly and accurately.
Default Value
This is a value that a user can enter into a field when a more appropriate value is not yet
available and nulls are disallowed. Use a default value very judiciously, and only if it is
meaningful.
For example, “WA” is a meaningful default value for a C
UST
S
TATE
field when
the vast majority of your customers live in Washington. Conversely, “01/01/12” is not a
