Cardinality

Owned by Computer Science Department
Last updated: Aug 05, 2023
2 min read

Cardinality in database design refers to the nature of relationships between tables in a database. Specifically, it describes the number of occurrences in one entity which are associated (or can be associated) with the number of occurrences in another.

Cardinality can be of different types:

  1. One-to-One (1:1): In a one-to-one relationship, one record in a table is associated with one and only one record in another table. For example, in a company database, each employee has one social security number, and each social security number is associated with one employee.

  2. One-to-Many (1:M): In a one-to-many relationship, one record in a table can be associated with one or more records in another table. But a record in the second table can be associated with only one record in the first table. For example, a customer can place many orders, but each order is placed by one and only one customer.

  3. Many-to-One (M:1): This is the opposite of a one-to-many relationship. One or more records in a table can be associated with one record in another table. For example, many students can be enrolled in the same course.

  4. Many-to-Many (M:N): In a many-to-many relationship, one record in a table can be associated with one or more records in another table, and vice versa. For example, a student can be enrolled in many courses, and each course can have many students.

Understanding the cardinality between tables is important when designing a database because it affects how tables are linked together. Many-to-many relationships, for example, typically require an additional "linking" or "junction" table to properly implement.

Also, understanding the cardinality of relationships can help optimize the performance of a database by allowing more efficient querying, data integrity, and use of storage resources.

In Entity-Relationship Diagrams (ERDs), cardinality is represented by crow's foot notation at the ends of a line linking two entities, which visually indicates the type of relationship (one-to-one, one-to-many, or many-to-many).