Database Reference
In-Depth Information
Further, this foreign key will form part of the primary key (or a candidate key) in the
component relation. For examples, please refer to figures 3-11 and 3.12 of chapter 3.
For components, integrity constraints on foreign (primary) keys may be:
•
Null FKs not allowed
•
Deletion of referenced records restricted (in summary)
•
Deletion of referencing records allowed (in component but not in
summary)
•
Update cascaded from the summary
5.5.7 Determining the Properties
The final step in this (modified) XR approach is to carefully determine the properties in
each relation (entity). This is actually easy, but in order to avoid mistakes, you must be
diligent:
•
Except for associations, in your initial system investigation (which
would be part of the required software engineering or systems
analysis), you would have identified the basic properties for each
identified entity. That is your starting point.
•
Next, go through steps 1-3, 5, and 6 above, and observe the
guidelines for dealing with characteristics, designations, subtypes
and components. These steps tell you when and where to
introduce foreign keys.
•
Next, observe step 4 above for treating associations.
By following this procedure, you will be able to confidently determine the properties
for each relation in the database; in fact, you will end up with a list that is identical or very
similar to the one provided in Figure 5-5 above. This finalized list is illustrated in Figure
5-8
.
As you examine the figure, please note the following:
1.
All primary key and foreign key attributes are italicized.
2.
The principle of having each attribute with a unique attribute
name (including foreign keys) has been followed.
3.
The database specification is presented via a Relation-
Attributes List (RAL) — a technique introduced in chapter 3
(section 3.6).
