Database Reference
In-Depth Information
this field
at some point
in the near future. After he's entered the value, he can then
edit it at any time.
3.
Enter Now, Edits Not Allowed:
A user
must
enter a value for this field when she
creates a new record in the field's parent table, but she
cannot
edit it at any time
whatsoever.
4.
Enter Later, Edits Not Allowed:
A user
has the option
of entering a value for this
field when he creates a new record in the field's parent table. This does not imply
in any way that the field's value can be null for all time; the user must enter a value
for this field
at some point
in the near future. After he's entered the value, he
can-
not
edit it at any time whatsoever.
You should use a default value when you set the Edit Rule element to the second or fourth
option; this will keep the field's value from being null until such time that the user enters
an appropriate value.
Comparisons Allowed
This indicates the types of comparisons a user can apply to a given field value when he's
retrieving information from the field. There are six types of comparisons: equal to (=), not
equal to (≠), greater than (>), less than (<), greater than or equal to (>=), and less than or
equal to (<=). This element also indicates whether a user can compare a given field value
to any of the following.
• Another value within the same field. When a field serves as a primary key, this op-
tion applies to the values of related foreign key fields. (You'll learn more about
this in the next chapter.)
• A value of another field within the parent table or from some other table in the
database.
• A value expression, which is some form of operation involving field values, literal
values, or a combination of both. It returns a single value that you can then use for
the comparison: (R
ETAIL
P
RICE
- 2.50) is an example of a value expression.
Controlling the types of comparisons a user can apply to the field's values enables you
to keep him from making meaningless comparisons. Let's say that he's working with an
E
MPLOYEE
ID N
UMBER
field based on a numeric data type. Unless you indicate otherwise,
he can make a comparison such as this one:
Is an Employee ID Number in the Employees table
greater than
or equal to
an Employee ID Number in the Part-Time Employees
table?
