A weak entity set and its identifying relationship, Database Management System

Q. Define a weak entity set and its identifying relationship. List the necessary conditions for a meaningful identifying relationship set of a weak entity set.   

Entity is a thing which can be distinctly identified it then goes on to classify entities into regular entities and weak entities, a weak entity is entity whtity which existence depend on some other  entity in the sense that it cannot exist  if  that other does not exist,  According to figure an employee's dependents might be weak entities they cannot exist if the relevant employee does not exist. Properties: Entities- also relationship have properties. All entities or relationship of a given type have certain kinds of properties in common; for example, all employees have an employee number, a name, a salary and so on. Each kind of property draws its values from a corresponding value set (in other words, a type or domain ). Furthemore properties can be: Simple or Composite: For example, the composite property" employee name" might be made up of the simple properties. "First name", "Middle initial" and "Last name. Key: Unique possibly with some context; for example, a dependent's name might be unique within the context of a given employee. Single or Multi-valued: All properties shown in figure are simple valued but if, for example, a given supplier could have several distinct location, then "supplier city" might be a multivalued property. Missing:Unknown or not applicable as illustrated in figure. Bace or Derived: For example, "total quantity" for a given part might be derived as the sum of the individual shipment quantities for that part. Relationship: Relationship is an association entities. For example, there is a relationship called DEPARTMENT-EMPLOYEE between departments and employees, representing the fact that certain departments are said to be the participants in that relationship. The number of participanrs in a given relationship is called the degree of the relationship. Sol.(b) the genetalization and sdecialization diagram for a given problem is as shown in the figure below At lowest level entities become the subtype or categories to the supertype. Generalization is a form of abstraction that specifies that two or mre entities that share common attributes can be generalezed into higher level entity type called a super type or generic entity. Here in this example the lowest level entities such as motor cycle car vans and buses have several common attributes which can be generalized into the entity type VEHICLE as shown in figure. Generalization process can be viewed as inverse of specialization process. Therefore in figure we can view as a specialization of VEHICLE rather than viewing VEHICLE as a generalization of CAR, VANS, BUSES and MOTOR CYCLE. 

 

Posted Date: 7/11/2012 4:03:28 AM | Location : United States






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