Database schema is the overall design of a database.
The database changes over time as data is inserted, updated or deleted.
Database instance is thus the data in the database at any particular point in time.
Data Independence
Data independence is the ability to modify a schema definition in one level without affecting a schema definition in next higher level.
Are of two types:
Logical
Physical
Logical
This is the ability to modify the conceptual schema without causing application programs to be rewritten.
Physical
The ability to modify the internal schema without having to change the conceptual or external schema.
It includes changes on storage devices, thus users should only note a change in performance.
Data Models
A model is a representation of the real world objects and events, and their association.
Data Model is the underlying structure of the database.
It is an integrated collection of concepts for describing and manipulating data, relationships between data and constraints on the data in an organization.
Data Model Categories
- Record Based
- Object Based
- Physical
Record Based
The database consists of a number of fixed format records of different types.
Each record type defines a fixed number of fields of a fixed length.
Are of three types:
i. Relational
ii. Network
iii. Hierarchical
