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

What is use of generating such tables, What is use of generating such table...

What is use of generating such tables? How can you create temporary tables? When you require a table only for a short time after that you want it to disappear automatically you

Tuples-attributes , Consider a relation s over the attributes A and B wit...

Consider a relation s over the attributes A and B with the following characteristics: 5,000 tuples with 10 tuples per page A 2-level B+ tree index on attribute

Create student db and perform delete operation, Create Student DB and perfo...

Create Student DB and perform delete operation? STUDENT (name, student#, class, major) COURSE (course name, course#, credit hours, department) SECTION (section identifier, co

Urgent need tutoring in conjunction with help on assignment!, Hi is anyone ...

Hi is anyone well versed in accounting information systems available now to help me work through an important assignment due tomorrow at noon?

External failure-database recovery and security, External failure : A failu...

External failure : A failure can also result because of an external cause, such as earthquakes, fire, floods, etc. The database must be duly backed up to avoid troubles occurring d

Implementing a distributed database system, A distributed database is defin...

A distributed database is defined as "a collection of multiple logically interrelated database which are physically distributed over a computer network" (a) Give any three advan

MANAGEMWNT OF PRINCILES AND TECHNIQUES., HOW TO SOLVE THE LP Problems by si...

HOW TO SOLVE THE LP Problems by simplex method?z=x1+-3x2+2x3

What is the use of integrity constraints, What is the use of integrity cons...

What is the use of integrity constraints? Integrity constraints make sure that changes made to the database by authorized users do not result in a loss of data consistency. The

What is data block, What is data block? Data Block - In oracle the data...

What is data block? Data Block - In oracle the data blocks are referred to as tablespaces. A tablespace is an fields of disk consisting of one or more disk files. A tablespace

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