Systems

ER

Updated at: 06 Aug 2019 07:51:47

Introduction

Conceptual model is used to communicate database designs. ER model is a conceptual model.

The database to be built will model a “Universe of Discourse”.

Components

  1. ER Modelling

  2. ER Diagrams

  3. ER Mapping

Entities

Entities can have attributes. All entities in one entity set have the same attributes. Attributes take different values for each entities.

Relationships

Relationship associates zero or more entities. A “relationship set” is a set of relationships associating entities from the same entity sets.

Relationships can associate entities from the same entity set. The relationship can be named. Relationships can have attributes. All relationships in one relationship set has the same attributes.

Entity identifiers: Key

One attribute can identify the entity, this is a property of all entities in an entity set. A combination of attributes can be used to identify the entity. In that case it would be called a composite key. If there are several possible candidates, it is called a candidate key.

Relationship Identifiers

by their participating entities.

Participation Constraints

Participation in r/s can be constrained by a minimum and maximum.