Foreign keys , Database Management System

Assignment Help:

Foreign Keys and NULL

Let us take the relation:                

DEPT

DEPT ID

DNAME

BUDGET

D1

Marketing

10M

D2

Development

12M

D3

Research

5M

 

EMP

EMP

ID

ENAME

DEPT

ID

SALARY

E1

Rahul

D1

40K

E2

Aparna

D1

42K

E3

Ankit

D2

30K

E4

Sangeeta

 

35K

Assume that Sangeeta is not assigned any section. In the EMP tuple corresponding to Sangeeta, thus, there is no genuine section number that can serve as the appropriate value for the DEPTID foreign key. Thus, one cannot verify BUDGET and DNAME for Sangeeta's department as those values are NULL. This may be actual circumstances where the person has newly joined and is undergoing training and will be allocated to a department only on finishing of the training. Thus, NULL in foreign key values might not be a logical error.

Thus, the foreign key definition may be redefined to conatin NULL as an acceptable value in the foreign key for which there is no require to find a matching tuple.

Is there any other constraint that may be applicable on the attribute values of the entities?

Yes, these constraints are mostly linked to the domain and termed as the domain constraints.


Related Discussions:- Foreign keys

System error-database errors, System error :These contain errors in databas...

System error :These contain errors in database system or the OS, example, deadlocks.Such errors are fairly hard to detect and need reprogramming the erroneous components of the sys

Replicating data, Replicating Data One approach that may support effici...

Replicating Data One approach that may support efficient access to dissimilar applications as above may be to give access to every of the applications from dissimilar replicate

B2b business uk database, We have a new website where I need someone to 1: ...

We have a new website where I need someone to 1: Inhabit the back end with all the recruitment agencies in the UK. I require someone who is not manually going to do this, but who w

Sql, Events4Fun is an event management company in Europe with branches in S...

Events4Fun is an event management company in Europe with branches in South America as well. The company is well-known for its efficiency, good-quality services, and affordable char

What are the states of transaction, What are the states of transaction? ...

What are the states of transaction? The states of transaction are:- a) Active b) Partially committed c) Failed d) Aborted e )Committed   f) Terminated

What is called a query -execution engine, What is called a query -execution...

What is called a query -execution engine? The query implementation engine takes a query evaluation plan, implements that plan, and returns the answers to the query.

Why do so various organizations use mysql, Why do so various organizations ...

Why do so various organizations use MySQL? Advantages: Open source Highly efficient Offers a high scalability in terms of size and connectivity. Highly prote

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