ER Diagram

Entity

An object or concept used to store information. Represented by a rectangle.

Attribute

Represents a characteristic or property of an entity. Represented by an oval. Entity and Attribute are connected with a line.

Attribute value

A particular value of the attribute.

Attribute domain

Set of possible values for an attribute.

Simple Attribute

Atomic. Cannot be further divided into smaller parts. Represented by a single-lined ovals.

Composite Attribute

When an attribute consists of 2 or more components. Each component may or may not be atomic. Each combination is denoted by an oval. Connected to the composite attribute with a line.

Multi-valued Attribute

Can have 1 or more values. When an attribute can have multiple values. Represented by a double-lined oval. Connected to the entity with a line.

Derived Attribute

Computed from other attributes. Represented by a dashed-lined oval connected to an entity with a dashed line.

Key Attribute

1 or more attribute(s) that can uniquely identify an entity instance. Can be composite.

Relationship

Relationship type

A set of associations between entity types.

Relationship instance

Ordered pair of 2 specific entities that are associated together. Usually drawn in a way that is read from left to right.

Relationship with attributes

Relationships can have attributes, which cannot be associated with a single entity.

Recursive Relationship

When the same entity participates in a relationship more than once. Two separate lines are used, each labelled with a different role, to represent the relationship.

Strong relationship

An entity is called strong if the existence of that attribute does not depend on other entity types.

Weak relationship

When an entity is not strong.

Cardinality

Number of instances of an entity type that can be associated with a relationship instance.

One-to-One

When only one instance of an entity type can be associated with the relationship.

One-to-Many

When the relationship between X, Y can have multiple instances of X associated with a single instance of Y, but Y is only associated with one instance of X.

Many-to-Many

When the relationship between X, Y can have multiple instances of X associated with a single instance of Y, and vice versa.