Explain generalization and aggregation in e_r diagram, Database Management System

Assignment Help:

Explain the concept of generalization and aggregation in E_R diagrams. Give one example for each one of them?

Generalization: Consider extending the entity set account through classifying accounts as being either savings-account or chequing-account.
Each of these is described through the attributes of account plus additional attributes. (Savings has interest-rate and cheque has overdraft-amount.)
We can express the similarities among the entity sets through generalization. This is the procedure of forming containment relationships among a higher-level entity set and one or more lower-level entity sets.

Aggregation: The E-R model cannot express relationships between relationships.
When would we require such a thing?

Consider a DB along with information about employees who work on a particular project and use a number of machines doing in which work. We get the E-R diagram shown in Figure.

620_Employee data.png

Figure: E-R diagram with redundant relationships

Relationship sets work and uses could be combined into a single set. Therefore, they shouldn't be, as this would obscure the logical structure of this scheme.
The solution is to use aggregation.
• An abstraction by those relationships is treated as higher-level entities.
• For our instance, we treat the relationship set work and the entity sets employee and project as a higher-level entity set called work.
• Another figure shows the E-R diagram with aggregation.

75_employee data2.png


Related Discussions:- Explain generalization and aggregation in e_r diagram

The conceptual level or global level in dbms, The Conceptual Level or Globa...

The Conceptual Level or Global level The conceptual level presents a logical view of the complete database as a unified whole. It permits the user to bring all the data in th

Constraints violation during insertion, If the insertion violates one or mo...

If the insertion violates one or more constraints, then two alternatives are available: Default option: - Insertion can be discarded and the reason of refusal can also be ex

Define the unique constraints for the table client, Define the following co...

Define the following constraints for the table client. (i) BAL_DUE must be at least 1000. (ii) NAME is a unique key. (i)ALTER TABLE CLIENT ADD CONSTRAINT CLIENT_BAL_DUE_C1

Online Bakery Data Base Project, How much would you charge to create a data...

How much would you charge to create a database encompassing an Online Bakery with all the elements below? I. Describe the application that you would like to design. This should

What is a super key, What is a super key? A super key is a set of one o...

What is a super key? A super key is a set of one or more attributes that collectively permits us to recognize uniquely an entity in the entity set.

Relationships, Let us first describe the term relationships. What Are Re...

Let us first describe the term relationships. What Are Relationships? A relationship can be defined as: A set of associations or connection, or A rule for communic

Query by example, Illustrate how you can use QBE to perform logical AND and...

Illustrate how you can use QBE to perform logical AND and OR operation

Erds, What are the various symbols used to draw an E-R diagram (any 3)?

What are the various symbols used to draw an E-R diagram (any 3)?

What is inheritance, What is inheritance? Define it in detail?  Inheri...

What is inheritance? Define it in detail?  Inheritance Inheritance can be at the levels of types, or at the level of tables we first consider inheritance of types, then i

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd