Database Reference
In-Depth Information
receives
SERVICE
PATIENT
ServiceCost
ServiceCost
PATIENT
SERVICE
Joe
Chest X-ray
75.00
Jane
Blood Test
95.00
Mary
Blood Test
65.00
Rob
MRI
325.00
Lisa
MRI
345.00
Figure 6-13
Example of an aggregate object.
fulfilled by
ORDER
VENDOR
PART
ship
WAREHOUSE
SHIPMENT
Quantity
ShipDate
Figure 6-14
Aggregate object: three-way relationship.
Relationships. An aggregate object may have relationships with other single
objects or other aggregate objects depending on the real-world information por-
trayed by the data model.
Under what circumstances do you have to show aggregate objects in your data
model diagram? If you find that a certain characteristic such as cost or date depends
on combinations of object instances, then an aggregate must be shown with that
characteristic as an attribute of the aggregate object. Again, if you note that
instances of some object are related to combinations of instances of other objects
and not to instances of single objects, then you need an aggregate object to show
the combination. Look at Figure 6-14 again for such a relationship.
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