Flashcards
Database Modelling
Cards in this deck (16)
- 1What is the relational model?An ER diagram is an example of a conceptual model. Elmasri and Navath (2011) describe the ER model as, entities, relationships and attributes. Elmasri and Navath (2011) continue saying an entity is a thing in the real world with an independent existence. In the case of Branson Railways this would be an employee or a customer.
- 2What are the limitation of the standard ER model?According to Batra, Bostrom and Hoffer (1990) however, the standard ER model suffers from shortcomings. Especially when trying to capture certain semantics of the real world.
- 3Any other limitations?Some forms of data require special representation such as temporal data, special data and multimedia data. The standard model cannot represent these effectively.
- 4List an example of a limitation.An example of this is the EMPLOYEE entity which can be of several types e.g. DRIVER, ADMIN and MANAGER. The standard ER model does not include methods to incorporate these into the system.
- 5What is EERD?EERD is a semantic data model. The EER model includes all the modelling concepts of the ER model. In addition, it includes the concepts of subclass and superclass and the related concepts of specialisation and generalisation.
- 6What are Superclass and Subclass ?An entity type EMPLOYEE describes the type of each employee entity, and also refers to the current EMPLOYEE entity in the Branson Railway database. In many cases an entity type has numerous subgroupings or subtypes of its entities that are meaningful and need to be represented explicitly because of their significance to the database application.
- 7List some examples of Superclass and Subclass.For example, the entities that are members of the EMPLOYEE entity type may be distinguished further into TRAIN DRIVER, SECRETARY, MANAGER, ADMIN, OFFICE_STAFF, DRIVER, and so on. Each of these subgroupings a subclass or subtype of the EMPLOYEE entity type. The EMPLOYEE entity type is called the superclass or supertype for each of these subclasses. This design more accurately reflects the Branson Railways scenario and correctly models the Universe of Discourse (UoD). This also eliminates duplication and minimises NULL values.
- 8What is Specialisation ?Specialization is the process of defining a set of subclasses of an entity type; this entity type is called the superclass of the specialisation. The set of subclasses that forms a specialisation is defined on the basis of some distinguishing characteristic of the entities in the superclass.
- 9Give an example of specialisation?The set of subclasses {ADMIN, SECRETARY, MANAGER} is a specialisation of the superclass EMPLOYEE that distinguishes among employee entities based on the job type of each employee entity.
- 10What is Generalisation?Generalisation is the opposite of specialisation and takes the common features of several entity types and generalises them into a single superclass.
- 11Give an example of Generalisation?For example, the entities DRIVER and OFFICE_STAFF share several common attributes. Therefore they can be generalised into the EMPLOYEE entity.
- 12what is Inheritance ?When a subclass has been identified, all the attributes are inherited from the superclass. For example he subclass DRIVER will inherit all the attributes from the entity EMPLOYEE. This is called attribute inheritance and is represented within the EERD with an arc.
- 13Give an example of Inheritance.A member entity of a subtype also inherits all the relationships of the corresponding entity in the supertype. However, a member entity may also have specific relationships for example, TRAIN STAFF are allocated a journey which only applies to the employee type TRAIN STAFF.
- 14List Database Design Problems.• Fan traps - arise from M:1 and 1:M relationships through one entity
• Chasm traps - two “many to one” joins converge on a single table
• Cyclic - Leads to redundant data
• Should include Temporal support - 15List other possible problems.The choice of architecture is also important when designing a database application. This can be achieved via a number of architectures for example, host machine, client-server, distributed or web interface. In the case of Branson railways a distributed architecture would be most appropriate. This is largely due to the nature of the business being distributed over the UK.
- 16EvaluationThe use of the enhanced ER model makes the design of the database simpler and easier to understand. The use of concepts such as inheritance, superclass and subclasses enables a more semantic approach to database design. In my opinion the use of relational mapping is also simpler to understand compared to normalisation, as the steps taken are more logical and are well defined. In my particular case I did not encounter any problems when the final tables were produced. I would use the tools and techniques I have learned in any future database design projects.
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- General questions about data modelleing.
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- adamelvin786
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