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

What are the uses of the information in the data dictionary, What are the u...

What are the uses of the information in the Data dictionary? The following information is directly taken from the Data dictionary: Information on fields displayed with

MYSQL, i have a homework assignment identical to one you have listed

i have a homework assignment identical to one you have listed

Database, Three way to handle missing field val

Three way to handle missing field val

Define two-phase locking protocol, Define two-phase locking protocol. A...

Define two-phase locking protocol. Ans: Two-phase locking is significance in the context of ensuring that schedules are serializable. The two-phase locking protocol defines a p

Explain relevance of data dictionary in a database system, Explain the rele...

Explain the relevance of Data Dictionary in a Database System? Data dictionary is a database in its own right residing on the disk that consist of Meta data which is = Data abo

Provide examples of different types of time and date formats, Provide Examp...

Provide Examples of different types of Time and Date formats.  It's most common store dates using a dash (-) as a colon (:)and the delimiter as the time delimiter. The given fo

NORRMALIZATION, Home based assignment Consider an ORDER table with the foll...

Home based assignment Consider an ORDER table with the following attributes: • Order Number (ONo),Customer Number (CNo),Customer Name (CNa),Item Number (INo),Unit Price (UP), Quant

Physical model of traffic data management, After the completion of the logi...

After the completion of the logical model the physical model is to be made. The oracle 10 G will be used as a database server and all the tables with their column are created and i

Give an example of object representation, Give an example of Object Represe...

Give an example of Object Representation  For illustration, consider the implementation of a social security number within an employee object that is shown in Figure. The soci

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