Databases Reference
In-Depth Information
or redundant expressions, and automatically create a conceptual schema for
a relational DBMS.
While text editors exist for entering text describing a conceptual schema
using English or some other natural language, it is not possible for a text
editor to check for missing or redundant descriptions. Nor is it possible to
automatically generate a relational conceptual schema from a conceptual
schema expressed in English. Therefore, English is rated low. SQL editors
and compilers enable DBAs to enter and check the completeness and redun-
dancy of conceptual schemas and to create a relational schema for use by a
relational DBMS. SQL receives a high rating. Graphic editors enable DBAs to
draw entity-relationship diagrams and relational diagrams. Some special-
ized graphic editors may even be able to check for missing or redundant
objects. Because these editors are not available for all relational DBMSs,
entity-relationship and relational diagrams are given a medium rating.
Many vendors of relational DBMSs also supply graphical editors to com-
pose entity-relationship and relational diagrams that automatically convert
the diagrams into SQL before being processed by a DBMS. These include:
• 4Keeps: A.D.Experts; Ann Arbor, MI; www.adexperts.com
• DataModeler: Iconix Software Engineering, Inc.; Santa Monica, CA;
www.iconixsw.com
• EasyCASE, Database Engineer, and EasyEr: Visible Systems Corp.;
Bellevue, WA; www.visible.com
• ER/Studio: Embarcadero Technologies, Inc.; San Francisco, CA;
www.embarcadero.com
• ERWin: Logic Works Inc.; Princeton, NJ; www.logicworks.com
• SilverRun Professional: SilverRun Technologies, Inc.; Woodcliff Lake,
NJ; www.silverrun.com
One more step is necessary to create a working database. The DBA must
create a physical schema to describe the data structures and access paths
that the DBMS will use to access a database. (When purchasing schema
design software, it is important to select product offerings that also enable
DBAs to specify the physical schema.)
RECOMMENDATION
Entity-relationship diagrams should be used to define conceptual sche-
mas. These diagrams directly represent many-to-many relationships, weak
entities, and class hierarchies. While the relational database equivalent of
these constraints can be represented using SQL or relational diagrams, the
understanding inherent in these structures is lost when they are converted
to their relational counterparts.
It is best to use a tool, such as a graphic editor, to capture entity-relationship
diagrams and to generate the equivalent SQL notation. This enables the
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