Kardinalitäten

Kardinalität refers to the number of times an instance of an entity can be related to an instance of another entity. In the context of database design and information modeling, it describes the relationships between tables or data elements. Understanding cardinality is crucial for ensuring data integrity and efficiency.

There are generally four types of cardinality: one-to-one, one-to-many, many-to-one, and many-to-many. A one-to-one relationship exists when a single instance of entity A can be related to at most one instance of entity B, and vice versa. For example, a person might have one passport, and a passport belongs to one person. A one-to-many relationship occurs when a single instance of entity A can be related to multiple instances of entity B, but an instance of entity B can only be related to one instance of entity A. An example is a customer who can place many orders, but each order belongs to only one customer.

A many-to-one relationship is the inverse of a one-to-many relationship, where multiple instances of entity A can be related to a single instance of entity B. A many-to-many relationship signifies that a single instance of entity A can be related to multiple instances of entity B, and a single instance of entity B can also be related to multiple instances of entity A. For instance, students can enroll in many courses, and a course can have many students. This type of relationship often requires an intermediate table to resolve it in relational databases.