Data aggregation, Database Management System

Assignment Help:

Q. Explain  data aggregation and discuss different design constraints.

Sol. Aggregation   One limitation of the E-R model is that it cannot express relationships among relationships. To illustrate the need for such a construct consider the ternary relationships works on which we will see between a employee, branch and job (see figure). Now, suppose we want to record managers for tasks performed by an employee at a branch that is we want to record managers for (employee branch job) combinations. Let us assume that there is an entity set manager. One alternative for representing these relationships is to create a quaternary relationship manages between employee, branch, job and manager (A quaternary relationships required a binary relationship between manager and employee would not permit us to represent which (branch, job) combinations of an employee are managed by which manager.) Using the basic E R modeling constructs we obtain the E R diagram of given figure.  It appears that the relationship sets works on and manages can be combined into one single relationship set. Nevertheless we should not combine them into a single relationship since some employee; branch job combinations may not have a manager.  There is redundant information in the resultant figure however since every employee; branch job combination it manages is also in works on. If the managers were a value rather than manager entity we could instead make manager a multivalued attribute of the relationship works on. But doing so makes it more difficult to find for example employee branch job triples for which a manager is responsible. Since the manager is manager entity this alternative is ruled out in any case.  The best way to model a situation such as the just described is to use aggregation. Aggregation is an abstraction through which relationship are treated as higher level entities. Thus for our example we regard the relationship set works on as a higher level entity set called works on. Such an entity set is treated in the same manager between works on and manager to represent who manages what tasks. Figure shows a notation for aggregation commonly used to represent the above situation. 

 

 


Related Discussions:- Data aggregation

By which techniquestransaction processing is associated, By which technique...

By which techniquesTransaction processing is associated? Transaction processing is associated by producing detail, summary, or exception reports and recording a business activi

DW, #ques Define a job scheduling strategy that will meet business requirem...

#ques Define a job scheduling strategy that will meet business requirement of reporting availability by 6am CST for the following cubes? Show the job scheduling dependencies in a p

Money making supermarket chain problem, Consider the ER schema below for th...

Consider the ER schema below for the MoneyMaking supermarket chain problem. Translate the given ER schema into SQL CREATE TABLE statement.

Explain data sublangauges, Explain Data Sublangauges ? Data sublanguage...

Explain Data Sublangauges ? Data sublanguage: In relational database theory, the term sublanguage, first used for this purpose through E. F. Codd in the year of 1970, refers t

Er schema, Consider entity type Employee with attributes empNumber (this is...

Consider entity type Employee with attributes empNumber (this is unique), empName, and the year when he/she was hired. An employee can either be a technical employee, a marketing e

Original Database, I need an ERD created for any original database(can be o...

I need an ERD created for any original database(can be on anything). It needs to be 3NF. I then need all the CREATE STATEMENTS, INSERT STATEMENTS(anything you can think of as long

Multi-key file organisation, Multi-Key File Organisation In this part, ...

Multi-Key File Organisation In this part, we will introduce two basic file Organisation schemes that permit records to be accessed by more than one key field, therefore, allowi

Explain the management of data store, Explain the Management of Data Store ...

Explain the Management of Data Store Every system irrespective of its nature of application requires storing permanent data for consequent use in problem solving. Some objects

Define garbage collection, Define garbage collection. Garbage may be fo...

Define garbage collection. Garbage may be formed also as a side effect of crashes. Periodically, it is essential to find all the garbage pages and to add them to the list of fr

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