DBS概念和組成 A database-management system (DBMS) is a collection of interrelated data and a set of programs to access those data. The collection of data, usually referred to as the database, contains information relevant to an enterprise. Data Abstraction Physical level. how the data are actually stored Logical level what information to keep in the database View level only a part of the database The user of the logical level does not need to be aware of this complexity. This is referred to as physical data independence. Instances and Schemas//數(shù)據(jù)庫實(shí)例和模式 The collection of information stored in the database at a particular moment is called an instance of the database. The overall design of the database is called the database schema. The physical schema describes the database design at the physical level, while the logical schema describes the database design at the logical level. A database may also have several schemas at the view level, sometimes called subschemas, that describe different views of the database.