Mapping Cardinalities (Dependencies) in DBMS

Mapping Cardinalities (Dependencies)

Cardinality defines the number of entities of one entity set, which are associated or join with the number of entities of another set via a relationship set.

One-to-one –

·         One entity from entity set A can be associated with only one entity of entity set B and vice versa.

• one student can enroll only for one course and a course will also have only one Student.

·         It is represented by adding ‘1’ near the entities on the line joining the relation.

·         In another type of notation one dash is added to the relationship line at both ends.

 

One-to-many –

·         One entity from entity set A can be associated with more than one entity of entity set B.

·         A one-to-many relationship is represented by adding ‘1’ near the entity at the left-hand side of the relation and ‘N’ is written near the entity on the right side.

· another type of notation will have a dash at LHS of relation and three arrow kind of lines at the RHS of relation as shown below.

1 student can opt for many courses,

 ·        Many-to-one –

o    More than one entity from entity set A can be associated with at most one entity of entity set B, however, an entity from entity set B can be associated with more than one entity from entity set A.

Student enrolls for only one Course but a Course can have many Students

 

·        Many-to-many –

o    One entity from A can be associated with more than one entity from B and vice versa.

one student can enroll in more than one course. And a course can have more than 1 student

·         A one-to-many relationship is represented by adding ‘M’ near the entity at the left-hand side of the relation and ‘N’ is written near the entity on the right side.

· another type of notation will have three arrow kinds of lines at both sides of relation as shown below.

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