Instances and Schemas Similar to types and variables in programming languages Schema-the logical structure of the database Example:The database consists of information about a set of customers and accounts and the relationship between them) Analogous to type information of a variable in a program Physical schema:database design at the physical level Logical schema:database design at the logical level Instance-the actual content of the database at a particular point in time Analogous to the value of a variable Physical Data Independence-the ability to modify the physical schema without changing the logical schema Applications depend on the logical schema In general,the interfaces between the various levels and components should be well defined so that changes in some parts do not seriously influence others Database System Concepts-5th Edition,May 23,2005 1.8 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 5 1.8 ©Silberschatz, Korth and Sudarshan th Edition, May 23, 2005 Instances and Schemas Similar to types and variables in programming languages Schema – the logical structure of the database Example: The database consists of information about a set of customers and accounts and the relationship between them) Analogous to type information of a variable in a program Physical schema: database design at the physical level Logical schema: database design at the logical level Instance – the actual content of the database at a particular point in time Analogous to the value of a variable Physical Data Independence – the ability to modify the physical schema without changing the logical schema Applications depend on the logical schema In general, the interfaces between the various levels and components should be well defined so that changes in some parts do not seriously influence others