Database Reference
In-Depth Information
These new entities help us relate the important pieces of data to one
another. After the basic entity list is in place, it is a matter of analyzing the
existing entities and their relationships to evaluate where there are holes in
the logical flow and storage of data. When you're trying to discover these
entities, it's helpful to ask yourself the following questions.
1. For every entity, are there attributes that apply sometimes, but not
always, to that entity?
The answer to this question will help you discover situations where
an entity's attributes are either too far reaching, or where you may
need to create a separate place to store the list of attributes that
may only occasionally apply to specific instances of the first entity.
2. For every entity, is there another entity that might have multiple
relationships to the entity being reviewed?
Obviously, this question helps you uncover many-to-many rela-
tionships.
3. For every entity, is there another type of data that should be stored
that isn't listed as a current entity?
This is more of a process or commonsense question. For example,
with MVM, it was obvious that we needed to store Shipments.
However, when we started thinking about attributes of a shipment,
it occurred to us that MVM uses multiple shipment methods and
multiple carriers, even though no one explicitly mentioned that in
the interviews. So while we were accounting for shipments, we
hadn't correctly identified all possible information relevant to that
process until we were reviewing our entity list.
We now have the complete list of entities for the MVM data model.
Next, we need to fill out the detailed information for each entity.
Attribute List
We now need to associate a list of attributes with every entity we've cre-
ated in order to define the data points that are being represented. This in-
cludes every attribute for all entities, with the exception of those that
define relationships; we cover those shortly.
As with the identification of the entities themselves, you extract the at-
tributes of each entity from the information you obtained during require-
ments gathering. You need to make sure that you have the definitive list of
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