Entity Relationship Diagram (ERD) — Part-II

Cardinality of a Relationship

Cardinality is the mapping of entities i-e zero, one or many. It basically explains how a table is linked to another table. It can be particularized more as the number of distinct values of a table connected to how many values of the other table – both minimum and maximum.

NOTE: Some authors explain optionality as a separate concept which is zero or one in this case.

Following are the two possible classifications of relations among entities.

• One to One
• One to Many

There is also a third case of (Many to Many) relationship but I consider it separate from the two basic classifications as it is basically resolved with the help of the same technique used in resolving (one to many) relationship. We will see that in detail later.

We will first discuss the One-to-Many and Many-to-Many  scenario as the example I am going to use is the One-to-One relation would need an understanding of one-to-many relation.