Advanced Database Management System - Tutorials and Notes: Describe cardinality ratios with example ERD
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Thursday, 2 March 2017

Describe cardinality ratios with example ERD

Define Cardinality ratio, Define Mapping cardinalities, What are the different types of relationships, Example ER diagrams for cardinality ratio

Cardinality ratios

In ER model, a relationship is an association among entities (records) of one or more entity sets. Cardinality ratio or mapping cardinalities is a concept that describes binary relationship set (a relationship that connects two entity sets) and its types. It is about the maximum number of entities of one entity set that are associated with the maximum number of entities of the other entity set.

For example, let us assume a student entity set (to model students’ personal information) and course entity set (to model the courses that are registered by a student). To establish a link (relationship) between these two entity sets we need to define the number of entities (records) of student that are associated with number of entities (records) of course. If our design requires that a student can register many courses then the relationship between student and course is a one-to-many relationship or the cardinality ratio between student and course is one-to-many.

See the example shown in the ERD below; [arrow head side - one side, without arrow – many side]

Figure: One-to-many relationship Registers from Student to Courses

The following are the types of relationship sets [cardinality ratios];
  • Many-to-one relationship
  • Many-to-many relationship


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