Cardinality

Every relationship has a cardinality. The cardinality defines how many destination records can potentially resolve the relationship. In relational database systems, there are generally two cardinalities:

• to-one relationship—for each source record, there is exactly one corresponding destination record
• to-many relationship—for each source record, there may be zero, one, or more corresponding destination records

An employeeDepartment relationship is an example of a to-one relationship: An employee can be associated with only one department in a company. An Employee entity might also be associated with a Project entity. In this case, there would be a to-many relationship from Employee to Project called projects since an Employee can have many projects.