RDBMS - Cardinality

Syllabus Detail

  • Cardinality in an RDBMS. 12 ATAR

What is Cardinality in RDBMS?

  • A relational database management system must have database tables that relate in some way
  • Cardinality refers to the way that one database table relates to another
  • When planning databases or systems with ER Diagrams, we use cardinality notation to solve relationships
  • There are three different types of cardinality; one-to-one, one-to-many and many-to-many
  • We often practice normalisation in order to resolve many-to-many cardinality relationships

 

Cardinality relationships:

  • One-to-one (1:1)
  • This occurs when two database tables directly relate to eachother
  • For example, if we have two tables; Drivers and Cars
  • In our database, only one driver may have one car at any time
  • Hence the relationship remains 1 (driver) : (drives) : 1 (car)
  • see here for why have a 1:1 relationship setup for security reasons
  • these are not very common

 

  • One-to-many (1:M)
  • This occurs when a table relates to many other tables in a database
  • For example, if we have two tables; Students and Assignments
  • In our database, one student can have submitted many assignments, but each one assignment is submitted by one student
  • Hence, the relationship remains 1 (student) : (submits) : M (assignments)
  • these are most common in an RDBMS

 

  • Many-to-many (M:N)
  • This occurs when many tables relate to many other tables in a database
  • This is virtually unachievable, hence why we use the process of normalisation to resolve many-to-many relationships
  • Read more about Normalisation by The Computing Teacher here

 

Further Research

  • Read more about cardinality by GeekInterview here
  • Degrees of Relationship by SQA here

 

Worksheet and Practice (yet to be added)

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