A set of attribute that describes the real world (e.g., employees) with attributes (e.g., employee_id)

• Entity set: A collection of similar entities

  • Each entity set has a key

  • Each attribute has a domain (type)

• Relationship: An idea of a relationship

  • Think of the relationship when creating views.

Constraints

• Key constraint:

  • One to Many: A key constraint on an entity such that each row of the entity can point to many rows of the other entity.

  • Many to Many: If there are no key constraint on an entity such that any row of the entity can point to any row of the other entity.

  • One to one: If there are key constraint on both entity then each row of each entity can only point to one.

  • One to many and one to one implies at most one

• Participation constraint:

  • A participation constraint on an entity such that every row must be part of the relationship to another entity.

  • This implies at least one.

Weak Entities

An entity owned by a primary key of the another (owner) entity that isn’t a full entity.

• Must have total participation.

• A weak entity has a partial key which can be a full key once it relates to its owner entity’s primary key.

Entity to Schema

• Key Constraints

  • One to One: FK(pk_1), FK(pk_2)

• Participation Constraint

  • Full participation = NOT NULL

• Weak entity

  • Composite key on id and the foreign key PK(pk, fk), FK(fk)