《数据库系统基本概念教学》appd.ppt

Appendix D: Network Model 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 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 type account = record customer-name: string; account-number: integer; customer-street: string; balance: integer; customer-city: string; 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 many-many, many-to-one, or one-to-one. 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. Data-Structure Diagrams (Cont.) For every E-R diagram, there is a corresponding data-structure diagram. Data-Structure Diagrams (Cont.) Since a link cannot contain any data value, represent an E-R relationship with attributes with a new record type and links. General Relationships To represent an E-R relationship of degree 3 or higher, connect the participating record types through a new record type that is linked directly to each of the original record types. 1. Replace entity sets account, customer, and branch with record types account, customer, and branch, respectively. 2. Create a new record type Rlink (referred to as a dummy record type). 3. Create the following many-to-one links: CustRlink from Rlink record type to customer record type AcctRlnk from Rlink record type to account record type BrncRlnk from Rlink record type to branch record type Network Representation of Ternary Relationship The DBTG CODASYL Model All links are treated as many-to