Sunday, 5 March 2017

Explain many-to-many relationship with example in DBMS

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

Many-to-many relationship

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

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

Example:
 
In a University database let us consider two entity sets Student and Course. Assume that a student can register many courses that are offered and a course may be registered by many students per semester. Then the relationship between Student and Course will be many-to-many.

Sample many-to-many relationship between entity sets

Observe carefully from the above figure the following;

  • Each student registered for zero or more courses [for example, Ravi registered for DBMS and Operating System]
  • Each course is registered by zero or more students [ for example, DBMS is registered by Ravi, Mark, and Mary]
The ER diagram for this case is shown below;

Many-to-many relationship Register between entity sets Student and Course

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