Databases Reference
In-Depth Information
UNDERSTANDABILITY AND USABILITY
Understandability and usability mean that the conceptual schema is
easy to learn, easy to understand, and easy to specify.
At first glance, English — or some other natural language — would appear
to be easy because no training is necessary to use it. However, natural lan-
guage sentences frequently can be vague and imprecise. Because of the
ambiguity and misunderstandings inherent in English and other natural lan-
guages, English has a low rating. The understandability of SQL is rated as
medium because users must carefully study the SQL descriptions in order to
visualize the conceptual model it describes. Entity-relationship diagrams
and relational diagrams receive a high rating because it is possible to com-
prehend the significant aspects of a conceptual schema by quickly examin-
ing the diagrams. Once again, the saying, “a picture” — or more appropri-
ately in this context, a diagram — “is worth a thousand words,” holds true.
Some DBAs rate the understandability of entity-relationship diagrams above
relational diagrams because the relationships reflect more of the diverse
structures of the conceptual schema than does the table-orientation of rela-
tional diagrams. On the other hand, the structure of relational diagrams is
familiar to relational DBMS users who may become confused when dealing
with the relationships of the entity-relationship CDDL.
Consistency
Consistency means that two DBAs will use the same constructs to
describe similar concepts. English receives a low rating because there may
be nearly an infinite number of ways to describe each and every business
rule. Entity-relationship diagrams are rated as medium because it is possi-
ble to describe many-to-many relationships as two one-to-many relation-
ships. It is also possible to describe an attribute of an entity set A as a weak
entity set that is related to A. SQL and relational diagrams are the most con-
sistent because they describe just a single type of thing — a table. There-
fore, SQL and relational diagrams receive a high rating.
Note that there might be more than one way to represent a collection of
attributes as tables. For example, a single table may contain all of the
attributes or multiple tables each may contain some of the attributes. DBAs
frequently use the process of normalization to break apart a table that rep-
resents multiple entity sets and structure the attributes into multiple tables.
Normalization occurs regardless of which CDDL is used. The concept of nor-
malization is orthogonal to the concept of consistency discussed here.
DBAs should perform normalization, regardless of which CDDL is used.
Software Support
Software support implies that software exists to enable DBAs to enter a
conceptual schema into the DBMS. Check the conceptual schema for missing
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