Levels of Abstraction Physical level:describes how a record (e.g.,instructor)is stored. Logical level:describes data stored in database,and the relationships among the data. type instructor record ID string; name string; dept name string; salary integer; end; View level:application programs hide details of data types.Views can also hide information(such as an employee's salary)for security purposes. Database System Concepts-6th Edition 1.7 @Silberschatz,Korth and Sudarshan
Database System Concepts - 6 1.7 ©Silberschatz, Korth and Sudarshan th Edition Levels of Abstraction Physical level: describes how a record (e.g., instructor) is stored. Logical level: describes data stored in database, and the relationships among the data. type instructor = record ID : string; name : string; dept_name : string; salary : integer; end; View level: application programs hide details of data types. Views can also hide information (such as an employee’s salary) for security purposes
View of Data An architecture for a database system view level view 1 view 2 view n logical level physical level Database System Concepts-6th Edition 1.8 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 1.8 ©Silberschatz, Korth and Sudarshan th Edition View of Data An architecture for a database system
Instances and Schemas Similar to types and variables in programming languages Logical Schema-the overall logical structure of the database Example:The database consists of information about a set of customers and accounts in a bank and the relationship between them Analogous to type information of a variable in a program 0 Physical schema-the overall physical structure of the database 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-6th Edition 1.9 @Silberschatz,Korth and Sudarshan
Database System Concepts - 6 1.9 ©Silberschatz, Korth and Sudarshan th Edition Instances and Schemas Similar to types and variables in programming languages Logical Schema – the overall logical structure of the database Example: The database consists of information about a set of customers and accounts in a bank and the relationship between them Analogous to type information of a variable in a program Physical schema– the overall physical structure of the database 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
Data Models A collection of tools for describing Data Data relationships Data semantics Data constraints Relational model Entity-Relationship data model(mainly for database design) Object-based data models(Object-oriented and Object-relational) Semistructured data model (XML) Other older models: Network model Hierarchical model Database System Concepts-6th Edition 1.10 @Silberschatz,Korth and Sudarshan
Database System Concepts - 6 1.10 ©Silberschatz, Korth and Sudarshan th Edition Data Models A collection of tools for describing Data Data relationships Data semantics Data constraints Relational model Entity-Relationship data model (mainly for database design) Object-based data models (Object-oriented and Object-relational) Semistructured data model (XML) Other older models: Network model Hierarchical model
Relational Model All the data is stored in various tables. Example of tabular data in the relational model Columns ID name dept_name salary 22222 Einstein Physics 95000 Rows 12121 Wu Finance 90000 32343 El Said History 60000 45565 Katz Comp.Sci. 75000 98345 Kim Elec.Eng. 80000 76766 Crick Biology 72000 10101 Srinivasan Comp.Sci. 65000 58583 Califieri History 62000 83821 Brandt Comp.Sci. 92000 15151 Mozart Music 40000 33456 Gold Physics 87000 76543 Singh Finance 80000 (a)The instructor table Database System Concepts-6th Edition 1.11 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 1.11 ©Silberschatz, Korth and Sudarshan th Edition Relational Model All the data is stored in various tables. Example of tabular data in the relational model Columns Rows