relationships

Degree

• number of attributes (when talking about relations)
  • for a table with 4 columns / attributes, the degree is 4
• number of entity type or entity sets associated (when talking about relationships)
  • types - unary, binary, ternary, n-nary
  • note: type of entities involved isnt same the number of entities (Student enrolled in a Course has a degree of 2, no matter how many students or courses are present)

Cardinality

• number of instances of one entity that can be associated with instances of another entity
• describes the relationship’s multiplicity
• types:
  • 1:1 - one instance of A can be mapped to one instance of B
  • 1:N - one instance of A can be mapped to N instances of B
  • M:N - M instances of A can be mapped to N instances of B
• implemented using primary & foreign keys, and join tables for M:N relationships

A relationship connects entities.
Cardinality describes how many instances of one entity relate to instances of another within that relationship.

Types of relationships:

• based on cardinality
  1. one to one (1:1)
  2. one to many (1:N)
  3. many to many (M:N)
• based on degree of relationship
  1. unary (degree: 1)
  2. binary (degree: 2)
  3. ternary (degree: 3)
  4. n-ary (degree: 4)
• based on participation
  1. total participation: all instances of the entity are involved in the relationship
  2. partial participation: only some instances of the entity are involved