Database Reference
In-Depth Information
Zachary's next task is to establish the appropriate relationship characteristics for each re-
lationship. He begins by defining a deletion rule for each relationship and then identifies
both the type of participation and the degree of participation for each table within the re-
lationship. He completes his task by designating these characteristics on the relationship
diagram.
Figure 10.69
shows one of the completed diagrams.
Figure 10.69. The completed relationship diagram for the EMPLOYEES and
INVOICES tables
Mike and Zachary review and verify all the relationships one last time. They agree that
everything is complete, so they celebrate with a couple of Mocha Bréves.
Summary
We opened this chapter with a discussion of the three types of relationships that can exist
betweenaparticularpairoftables—
one-to-one, one-to-many,
and
many-to-many.
Younow
knowthat the one-to-many relationship isthe most common type ofdual-table relationship
and that the many-to-many relationship gives rise to problems that must be resolved. You
then learned about a
self-referencing relationship,
which is a type of relationship that oc-
curs between the records within a given table. It is similar to a dual-table relationship in
that it can be one-to-one, one-to-many, or many-to-many.
Next, we discussed how to
identify
the relationships that exist among the tables in a data-
base. First you learned how to construct and use a
table matrix,
and then you learned how
to use
associative
and
contextual
questions to help you identify a given relationship. We
then discussed three formulas you could use to determine the true relationship that exists
between the tables in a dual-table relationship or between the records in a self-referencing
relationship.
The chapter continued with a discussion of how relationships are
established.
You learned
that one-to-one and one-to-many relationships are established by using
primary keys
and
foreign keys,
and that many-to-many relationships are established using
linking tables.
We
