Please visit, subscribe and share 10 Minutes Lectures in Computer Science

Thursday, 2 March 2017

Explain one-to-many relationship with example in DBMS

What is one-to-many relationship in DBMS, one-to-many relationship example, cardinality ratio one-to-many

One-to-many relationship

If an entity [a record] of one entity set is associated with zero or more entities of the other entity set, then the cardinality ratio is said to be one-to-many from one side entity set to the many side entity set.

When we would say the relationship is one-to-many?
Assume two entity sets A and B. The relationship is one-to-many from A to B if and only if “an entity in A is associated with zero or more entities (records) in B and an entity in B is associated with only one entity (record) in A”.
If we put in simpler terms, entity set B is the many side for A and entity set A is the one side for B.

Let us assume a database for Airline Reservation System. Further assume that there are two entity sets Flights and Flight_Attendants to model the flight details and the flight attendants details respectively. If one flight can have many attendants, then this relationship can be modeled as one-to-many from Flight to Flight_Attendants.
When we say that the relationship type is either one-to-many or many-to-one, it is always important to say which side is one and which side is many to avoid ambiguity.
Sample one-to-many relationship between entity sets

Observe carefully from the above figure the following;
  • Each flight is having one or more attendants. [look from flight to flight_attendant]
  • Each flight attendant is allotted with exactly one flight. [look from flight_attendant to flight]
The ER diagram for this case is shown below;
One-to-many relationship Has between entity sets Flight and Flight_Attendant

This can be read as, one flight has many flight attendants and one flight attendant is attending only one flight.

No comments:

Post a Comment

Featured Content

Multiple choice questions in Natural Language Processing Home

MCQ in Natural Language Processing, Quiz questions with answers in NLP, Top interview questions in NLP with answers Multiple Choice Que...

All time most popular contents