What is the number of instances of one entity that can or must be associated with each instance of another entity?

person, place, object, event, or concept in the user environment about which the organization wishes to maintain data

Entity type (collection of entities) often corresponds to a table

Entity instance (single occurrence of an entity) often corresponds to a row in a table

entities are not the same as system inputs and outputs (e.g., forms, reports)

entities are not the people that use the system/data

property or characteristic of an entity type that is of interest to the organization

identifier attribute uniquely identifies an entity instance (i.e., a key)

describes the link between two entities

usually defined by primary key - foreign key equivalencies

Refers to the number of instances of entity A that can be associated with each instance of entity B

One-to-one: Each entity in the relationship will have exactly one related entity

One-to-many: An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maximum of one related entity

Many-to-many: Entities on both sides of the relationship can have many related entities on the other side

Cardinality Constraints - the number of instances of one entity that can or must be associated with each instance of another entity.