Explain one-to-one relationship with example, one-to-one relationship in ER diagram, Define one-to-one relationship
One-to-one relationship
If an entity [a record] of one entity set is
associated with maximum
of one entity of the other entity set, then the relationship type is
said to be one-to-one.
When we would say the relationship is
one-to-one?
Assume two entity sets A
and B. The relationship between A and B is one-to-one if and only if “an entity in A is associated with only
one entity (record) 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 one side for A
and entity set A is the one side for B.
|
Example:
Let us assume a database for Airline
Reservation System. Further assume that there are two entity sets Passengers
and Boarding_Pass to model the passenger details and the flight tickets details
respectively. If it is decided to issue each passenger a boarding pass, then
the relationship between the entity sets Passengers and Boarding_Pass will be
One-to-one.
As shown in the figure
below, each passenger is allotted only one seat. Each passenger entity is
associated with exactly one boarding_pass entity. |
| Sample one-to-one relationships showing links between entities of two entity sets |
Observe carefully from the above figure the
following;
- Each passenger is allotted with exactly one seat. [look from passenger to boarding_pass]
- Also, each seat is allotted to exactly one passenger. [look from boarding_pass to passenger]
The ER diagram for this is as follows;
 |
| One-to-one relationship between two entity sets Passengers and Boarding_Pass |
Look at the ERD carefully. Arrow head is used
in either side of the relationship Given. It indicates, one passenger for one
boarding_pass, also one boarding_pass for one passenger.
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