Define Cardinality ratio, Define Mapping cardinalities, What are the different types of relationships, Example ER diagrams for cardinality ratio
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|
- Many-to-one relationship
- Many-to-many relationship
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