Entity-Relationship Models

  • 11 sept. 2024
  • 27 sept. 2024
  • 1 min de lecture
  • llms.txt

E/R model

sketch databse schemas including constraints.

  • Entity set = rectangle
  • Attribute = oval, with a line to the rectangle (representing its entity set)

relationship

  • connects two or more entity sets.
  • represented by a diamonds

value of a relationship is a relationship set

many-to-many relationship

an entity of either set can be connected to many entities of the other set.

many-to-one relationship

each entity of the first set can be connected to at most one entity of the second set. and each entity of the second set can be connected to at least one entity of the first set.

Liens retour

databases internals
Practice Q1. a. F b. F (wrong: Must be T) A relation R(A,B,C) may have at most three minimal keys (not superkey) c. T d. T e. T (any ops involving a null is a null) f.

Entity-Relationship Models

E/R model

sketch databse schemas including constraints.

  • Entity set = rectangle
  • Attribute = oval, with a line to the rectangle (representing its entity set)

relationship

  • connects two or more entity sets.
  • represented by a diamonds

value of a relationship is a relationship set

many-to-many relationship

an entity of either set can be connected to many entities of the other set.

many-to-one relationship

each entity of the first set can be connected to at most one entity of the second set. and each entity of the second set can be connected to at least one entity of the first set.