tailieunhanh - Lecture Database system concepts (6/e): Appendix D - Silberschatz, Korth, Sudarshan

Appendix D - Network model, cover the network and hierarchical data models. Both these data models predate the relational model, and provide a level of abstraction that is lower than the relational model. They abstract away some, but not all, details of the actual data structures used to store data on disks. These models are only used in a few legacy applications. | Appendix D: Network Model Database System Concepts, 6th Ed. ©Silberschatz, Korth and Sudarshan See for conditions on re-use Appendix D: Network Model Basic Concepts Data-Structure Diagrams The DBTG CODASYL Model DBTG Data-Retrieval Facility DBTG Update Facility DBTG Set-Processing Facility Mapping of Networks to Files Database System Concepts - 6th Edition ©Silberschatz, Korth and Sudarshan Basic Concepts Data are represented by collections of records. similar to an entity in the E-R model Records and their fields are represented as record type type customer = record customer-name: string; customer-street: string; customer-city: string; type account = record account-number: integer; balance: integer; end end Relationships among data are represented by links similar to a restricted (binary) form of an E-R relationship restrictions on links depend on whether the relationship is manymany, many-to-one, or one-to-one. Database System Concepts - 6th Edition ©Silberschatz, Korth and Sudarshan Data-Structure Diagrams Schema representing the design of a network database. A data-structure diagram consists of two basic components: Boxes, which correspond to record types. Lines, which correspond to links. Specifies the overall logical structure of the database. Database System Concepts - 6th Edition ©Silberschatz, Korth and Sudarshan Data-Structure Diagrams (Cont.) For every E-R diagram, there is a corresponding data-structure diagram. Database System Concepts - 6th Edition ©Silberschatz, Korth and .