Database Systems: Concepts, Design and Applications

2.5. Mappings

The three schemas and their levels discussed in Section 2.3 are the description of data that actually exists in the physical database. In the three-schema architecture database system, each user group refers only to its own external schema. Hence, the user’s request specified at external schema level must be transformed into a request at conceptual schema level. The transformed request at conceptual schema level should be further transformed at internal schema level for final processing of data in the stored database as per user’s request. The final result from processed data as per user’s request must be reformatted to satisfy the user’s external view. The process of transforming requests and results between the three levels are ...