Database Reference
In-Depth Information
1.
Mandatory:
There must be
at least one
record in this table before you can enter
any records into the related table.
2.
Optional:
There is no requirement for
any
records to exist in this table before you
can enter records into the related table.
You'll commonly determine the type of participation for most tables later when you're de-
fining business rules, although you can quite often establish the type of participation for
tablesinrelationshipswherethetypeofparticipationforeachtableisobvious,isaresultof
commonsense,orisinaccordancewithsomeparticularsetofstandards.Forexample,con-
sidertheone-to-manyrelationshipbetweentheEMPLOYEESandCUSTOMERStablesin
Figure 10.59
.
(These are slightly different versions of the tables in
Figure 10.56
.)
Figure 10.59. What type of participation should you assign to each table?
Assume that each customer must be assigned to a particular employee. This employee acts
as the customer's account representative and takes care of all transactions and communica-
tions between the organization and that customer. Although each customer
must
be associ-
ated with a particular employee, a given employee does not have to be associated with any
customer at all. Many employees perform other functions within the organization that do
not require customer interaction.
This scenario neither implies nor defines any special circumstances, but does indicate the
manner in which the organization conducts this part of its business. As such, you can infer
the following.
•
You should designate a Mandatory type of participation for the EMPLOYEES
table.
This ensures that there is at least one employee for you to assign to a given
customer.
•
You should designate an optional type of participation for the CUSTOMERS table.
This allows you to enter any person employed by the organization.
Once you've determined the type of participation for each table within the relationship,
designate each table's participation on the relationship diagram. Use a vertical line to rep-
resent a Mandatory type of participation and a circle to represent an optional type of par-
