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A schema is an overall description of a database and it is usually represented by the Entity Relationship diagram. There are multiple subschemas which are representations of the external models. To put it into perspective of one database:
- External Schemas :
- multiple multiple subschemas = multiple external views of the data •
- Conceptual Schema : one only
- Data items, relationships & constraints – ER diagram
- Physical Schema : one only
- Record definitions, representation methods
- Access methods, indexes, hashing
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