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

Aggregation, Design a database for an airline.The database must keep track ...

Design a database for an airline.The database must keep track of customers and their reservations,flights and their status, seat assignment on individual flights and, the schedule

Incident report to the database, The Form input elements are:           ...

The Form input elements are:             Reporter name                                                text type input             Password

Define page, Define page. The database is partitioned into some number ...

Define page. The database is partitioned into some number of fixed-length blocks, which are referred to as pages.

Create a data flow diagram, For your chosen project, you need to identify t...

For your chosen project, you need to identify the current problem that you wish your project to solve.  Produce a Problem Statement.  The Problem Statement should explain the purpo

Entity-relationship diagram, 1. Item based services will be charged fixed p...

1. Item based services will be charged fixed prices, and time-based will be charged variably depending on time. Such as per hour. Item-based: Fixed, pedicure, waxing, threading, fa

Display how records can be deleted and updated in qbe, With the help of an ...

With the help of an example display how records can be deleted and updated in QBE. (i) Increase Pay_Rate of employees along with the skill of ‘cook' by 10%. (ii) Delete employee

Give the levels of data abstraction, Give the levels of data abstraction? ...

Give the levels of data abstraction? There are three levels of Data Abstraction a) Physical level b) logical level c) view level

Why we use delegation to share implementation, Use Delegation to Share Impl...

Use Delegation to Share Implementation The delegation " Delegation comprises of catching an operation on one object and sending it to other object which is part, or related to

What are the reasons for having variable length records, What are the reaso...

What are the reasons for having variable length records? What are the various ways to store variable length records? Variable-length records arise in a database in various wa

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