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| Re: Why diffrrent Logical and Physical structures [message #103596 is a reply to message #103562] |
Tue, 27 January 2004 14:14   |
William Robertson
Messages: 1643
Registered: August 2003
Location: London, UK
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Senior Member |
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A logical model (entity model, entity relationship diagram) is independent of implementation or vendor. It represents at a high conceptual level the items (entities) relevant to the business and the relationships between them. In the logical model you will not be concerned with partitioning, index-organized tables, bitmap indexes and so on. You may also represent many-to-many relationships and super/subtypes, which would need to be resolved for the physical model.
The physical model deals with real tables and their actual implementation. For the physical model you need to know whether it will be implemented in Oracle, SQL Server, MySQL etc, as this affects the storage options, indexing, datatypes, foreign key definitions etc.
A common error is to confuse entities and tables. An entity is a concept in a logical model that allows you to say things like "A Doctor may have one or more Patients", DOCTOR and PATIENT being entities. In an actual database these will be implemented as tables such as DOCTORS and PATIENTS.
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