2.4. Describe the three-schema architecture. Why do we need mappings between
schema levels? How do different schema definition languages support this architecture?
Defines DBMS schemas at three levels:
Internal schema at the internal level to describe physical storage structures and access paths (e.g indexes).
Typically uses a physical data model.
Conceptual schema at the conceptual level to describe the structure and constraints for the whole database for a community of users.
Uses a conceptual or an implementation data model.
External schemas at the external level to describe the various user views.
Usually uses the same data model as the conceptual schema.
2.5. What is the difference between logical data independence and physical data independence?
Logical Data Independence:
The capacity to change the conceptual schema without having to change the external schemas and their associated application programs.
Physical Data Independence:
The capacity to change the internal schema without having to change the conceptual schema.
For example, the internal schema may be changed when certain file structures are reorganized or new indexes are created to improve database performance