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







Related Discussions:- A weak entity set and its identifying relationship, Assignment Help, Ask Question on A weak entity set and its identifying relationship, Get Answer, Expert's Help, A weak entity set and its identifying relationship Discussions

Write discussion on A weak entity set and its identifying relationship
Your posts are moderated
Related Questions
To do the division operation R1 ÷ R2, R2 should be a correct subset of R1. In the following example R1 have attributes A and B and R2 have only attribute B so R2 is a correct subse

What is data definition language? Explain it in detail? The SQL DDL allows specification of not only a set of relations, but also information after each relation, containing

Give the forms of triggers? The triggering event can be insert or delete. For updated the trigger can state columns. The referencing old row as clause The referencing

What are the objectives of query processing?       Ans: The objectives of query processing is to transform a query written in a high level language into a accurate and efficien

What is a transaction ? The Transaction is a unit of data processing. For instance, many of the transactions at a bank may be withdrawal or deposit of money; transfer of money f

Partial Key : It is a group of attributes that can uniquely identify small entities and that are related to same base entity. It is sometime named as Discriminator.  Altern

In order to keep pace with competitors, Space Ltd has decided to implement an ERP system in order to replace its old legacy and manual systems. Management does not have much knowle

Section A:  Use the following tables to create a database called College.  Use SQL commands. Student stuid(primary) lastName firstName

An object-oriented design process  1. Define the context and modes 2. Designs the system architecture 3. Identifies the principal system objects 4. Identifies concurrency in

What is the use of a slotted-page structure and what is the information are there in the header? The slotted-page structure is used for organizing records within a one block.