The ability of modifying a schema definition in one level without affecting a schema definition in a Higher Level is called Data Independence.
Logical Data Independence
- The ability to modify the conceptual scheme without causing application programs to be re-written.
- Immunity of external schemas to change in conceptual schema
- Usually done when logical structure of database is altered
Physical Data Independence
- The ability to modify the internal schema without having to change the conceptual or external schemas.
- Modifications at this level are usually to improve performance.
Schema of a database system is its structure described in a formal language supported by the database management system (DBMS)
- Refers to the organization of data to create a blueprint of how a database will be constructed (divided into database tables).
- The formal definition of database schema is a set of formulas (sentences) called integrity constraints imposed on a database.