Table Relationships - Database Design for Mere Mortals - page 307

have mistakenly entered employee ID number “100” for a given order when
you meant to enter “110.”
Figure 10.54 shows an example of these modifications for the E MPLOYEE ID
N UMBER foreign key field. (Note the setting for the Range of Values—this is one
good way to set this element.)
Figure 10.54. Logical Elements for the E MPLOYEE ID N UMBER foreign key field in the
ORDERS table
In order for you to see the significance of these modifications, Figure 10.55 shows
the Logical Elements category from the Source Specification. (Recall that this ele-
ment is in the General Elements category; see Figure 10.53 .)
• It draws its values from the primary key to which it refers. By definition, a foreign
key's range of values is limited to existing values of the primary key to which it
refers. For example, you cannot enter an invalid E MPLOYEE ID N UMBER into the
ORDERS table. Any E MPLOYEE ID N UMBER you enter into the ORDERS table
must first exist as an E MPLOYEE ID N UMBER in the EMPLOYEES table. This en-