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Terminology (pertaining to "relationship")

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• Definitions: (1) Relationship Instances, (2) Relationship Sets and (3) Relationship Types

  • A relationship instance among n entity types E₁, E₂, ..., E_n is:

    Any (one) tuple from the cartesian product E1 × E2 × ... × En

    Example:

    E1 = {John, James, Mary} E2 = {CS170, CS255, CS377, CS450} Then: (John, CS170) is a relationship instance (it could mean that "John took CS170")

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  • A relationship set is:

    the set (= collection) of relationship instances at a given moment in time.

    Example:

    E1 = {John, James, Mary} E2 = {CS170, CS255, CS377, CS450} Then: {(John, CS170), (John, CS255), (Mary, CS170)} is a relationship set

    (A relationship set is the same as a relation in Mathematics)

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  • A relationship type R :

    defines the association (= description of a relationship) among the entity types E1, E2, ..., En.

    Example:

    E1 = {John, James, Mary} E2 = {CS170, CS255, CS377, CS450} "Enrollment" relationship type between E1 and E2 is: A student ∈ E1 is enrolled in a course ∈ E2

    We call a relationship type: a relationship for short.

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• Summary and example
  • Summary:

    Relationship type (relationship for short): association between entities Example: Enrollment relationship type represents: Student enrolled in Course Relationship instance = a tuple from the cartesian product that represents the relationship Example: ( John, CS170 ) Relationship set = a set of tuples from the cartesian product that represents the relationship Example: {(John, CS170), (John, CS255), (Mary, CS170)}