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Data
Architect Professional database modeling |
Entity Relationship Diagram (ERD)
An ERD is a relatively simple diagram which shows the central elements of a database and the relationships those elements have to each other. The most used database element is the Entity and a Relationship. The Entity is typically represented in the diagram, as some form of box while the Relationships are some form of line connecting two Entities.The Logical Model is a type of ERD which depicts the database structures without regard to the specific database for which it will ultimately be implemented in. This allows for database design in the very early stages of system development and provides a way to 'flesh-out' the design without the encumbrances associated with implementation details. The LM, once complete, is morphed into a Physical Model where it can be optimized for a specific database.
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figure 1 Logical Model |
The Physical Model is a type of ERD which deals with a specific implementation of a database design. For example; the PM may be for a MySQL or a DB2 database. The PM is database specific because it often uses features, such as particular data types, only found in the specific database. The database elements are usually referred to by slightly different names in a PM as the following chart shows;
Logical Model |
Physical Model |
Description |
Entity |
Table |
a logical grouping of related data |
Relationship |
Reference |
shows the relationship between two entities
or tables |
Attribute |
Column |
also known as a Field |
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figure 2 Physical Model |