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.(变量类型)
Instance is analogous to the value of a variable.(变量取值)
Schema(模式)
Schema is the overall design of the database.(数据库的整体设计)
Several schemas of DBS
- Physical schema(物理模式/内模式/存储模式): database design at the physical level.
- Logical schema(逻辑模式/模式/概念模式): database design at the logical level.
- Sub schema(子模式/外模式): describe different views at the view level.
@see level of data abstraction
Physical data independence(物理数据独立性)
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(实例)
An instance is the collection of information stored in the database at a particular point in time.(特定时刻存储在数据库中的信息的集合)