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・ Network Kanji Filter
・ Network Knowledge
・ Network layer
・ Network length (transport)
・ Network Level Authentication
・ Network literacy
・ Network Load Balancing
・ Network Load Balancing Services
・ Network management
・ Network management application
・ Network management station
・ Network manager
・ Network mapping
・ Network media
・ Network medicine
Network model
・ Network model (disambiguation)
・ Network monitoring
・ Network monitoring interface card
・ Network motif
・ Network Neram
・ Network News Transfer Protocol
・ Network NorthWest
・ Network Norway
・ NetWork Oasis
・ Network of African National Human Rights Institutions
・ Network of African Science Academies
・ Network of Asian River Basin Organizations
・ Network of Associations of Local Authorities of South-East Europe
・ Network of Bay Area Worker Cooperatives


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Network model : ウィキペディア英語版
Network model

The network model is a database model conceived as a flexible way of representing objects and their relationships. Its distinguishing feature is that the schema, viewed as a graph in which object types are nodes and relationship types are arcs, is not restricted to being a hierarchy or lattice.
==Overview==
While the hierarchical database model structures data as a tree of records, with each record having one parent record and many children, the network model allows each record to have multiple parent and child records, forming a generalized graph structure. This property applies at two levels: the schema is a generalized graph of record types connected by relationship types (called "set types" in CODASYL), and the database itself is a generalized graph of record occurrences connected by relationships (CODASYL "sets"). Cycles are permitted at both levels.
The chief argument in favour of the network model, in comparison to the hierarchic model, was that it allowed a more natural modeling of relationships between entities. Although the model was widely implemented and used, it failed to become dominant for two main reasons. Firstly, IBM chose to stick to the hierarchical model with semi-network extensions in their established products such as IMS and DL/I. Secondly, it was eventually displaced by the relational model, which offered a higher-level, more declarative interface. Until the early 1980s the performance benefits of the low-level navigational interfaces offered by hierarchical and network databases were persuasive for many large-scale applications, but as hardware became faster, the extra productivity and flexibility of the relational model led to the gradual obsolescence of the network model in corporate enterprise usage.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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