Database-MySQL [3i Infotech Placement]: Sample Questions 91 - 91 of 162

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Question 91

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Describe in Detail

Essay▾

Brief theory of Network, Hierarchical schemas and their properties.

Explanation

Network schemas:

Given Network Model
  • Network model is a database model as a flexible way for a representing objects and their relationship.
  • Multiple parent and child records, the network model allows each record to have, forming a generalized graph structure.
  • This property applies at two levels: the schema is a generalized graph of record types connected by relationship types and the database itself is a generalized graph of record occurrences connected by relationships.
  • 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 disappearance of the network model in corporate enterprise usage.

Hierarchical schemas

  • It was an improvement on general file-processing systems because it allows the creation of logical relationships between information in a database.
  • The disadvantages of this structure is that led to the eventual development of the network model and the relational model.
  • The treelike structure is main characteristic of a hierarchical data model.
  • For example, a company database might organize using one branch for Staff, followed by Departments, Teams and then Team Members.
  • This structure is containing parent-child, it consistent throughout the database, and each child segment can only have one parent segment.
  • Each segment, or record, can have any number of field elements giving information on that record.
  • For example, the team member record would have details like name, supervisor and contact details.

One-to-Many and Redundancy:

  • This can lead to redundancy in the model.
  • For example, a branch below Team Members might be called Ongoing Projects.
  • Multiple staff members may work on one project, the project information must be duplicated, possibly leading to consistency issues.

Navigation:

  • The navigational data model is a hierarchical data model- the access paths in the model are predetermined structures.
  • The query moves from the root segment in the database down through the branches, to take a specific file record
  • This is fine if you already know the location of the records you seek, but if you are making find queries, this is slow, as the database must read all the records on a given level before moving to the next one.

Logical Parent Pointers:

  • The limitations of the hierarchical structure are somewhat by using logical parent pointers.
  • This involves setting up a new database for entries that have many-to-many relationships and linking the two, developed by IBM in their Information Management System data model
  • For example, the Ongoing Projects branch would have pointers that link the user to a separate Projects database where project information is contained.
  • This is similar to how the XML Extensible Markup Language IDREF function works.

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