Relationship between schema and instance
It is similar to types and variables in programming languages.
Schema is analogous to type information of a variable in a program. ( Variable type)
Instance analogous to IS The value of variable A. (Variable value)
Schema (mode)
Schema is the overall design of the database. (The overall design of the database)
Several schemas of DBS
- Physical schema (physical mode / intra modes / storage mode): The Database Design AT PHYSICAL Level .
- Logical schema (logical pattern / mode / conceptual models): The AT Database Design Logical Level .
- Sub schema (sub-mode / external mode): Different views AT DESCRIBE The View Level .
@see level of data abstraction
Physical data independence (physical data independence)
Application do not depend on physical schema, and thus need not be rewritten if the physical schema changes.
- Applications depend on the logical schema.
- Physical schema is hidden beneath the logical schema.
Instance (instance)
The Collection instance of IS AN Stored in Information Database The AT A Particular Point in Time . (At a particular time set information stored in the database)
