Diagram Illustration

Contents

Entity-relationship diagrams (ERD) are essential to modeling anything from simple to complex databases, but the shapes and notations used can be very confusing. This guide will help you to become an expert in ER diagram notation, and you will be well on your way to model your own database!

Conceptual ER diagram symbols

Conceptual Data Models establish a broad view of what should be included in the model set. Conceptual ERDs can be used as the foundation for logical data models. They may also be used to form commonality relationships between ER models as a basis for data model integration. All of the symbols shown below are found in the UML Entity Relationship and Entity Relationship shape library of Lucidchart.

ERD entity symbols

Entities are objects or concepts that represent important data. Entities are typically nouns such as product, customer, location, or promotion. There are three types of entities commonly used in entity relationship diagrams.

Entity Symbol Name Description

Strong Entity Symbol

Strong entity

These shapes are independent from other entities, and are often called parent entities, since they will often have weak entities that depend on them. They will also have a primary key, distinguishing each occurrence of the entity.

Weak Entity Symbol

Weak entity

Weak entities depend on some other entity type. They don't have primary keys, and have no meaning in the diagram without their parent entity.

Associative entity symbol

Associative entity Associative entities relate the instances of several entity types. They also contain attributes specific to the relationship between those entity instances.

ERD relationship symbols

Within entity-relationship diagrams, relationships are used to document the interaction between two entities. Relationships are usually verbs such as assign, associate, or track and provide useful information that could not be discerned with just the entity types.

Relationship Symbol Name Description

Relationship Symbol

Relationship

Relationships are associations between or among entities.

Weak Relationship Symbol

Weak relationship

Weak Relationships are connections between a weak entity and its owner.

ERD attribute symbols

ERD attributes are characteristics of the entity that help users to better understand the database. Attributes are included to include details of the various entities that are highlighted in a conceptual ER diagram.

Attribute Symbol Name Description

Attribute Symbol

Attribute

Attributes are characteristics of an entity, a many-to-many relationship, or a one-to-one relationship.

Multivalued Attribute Symbol

Multivalued attribute

Multivalued attributes are those that are can take on more than one value.

Derived Attribute Symbol

Derived attribute

Derived attributes are attributes whose value can be calculated from related attribute values.

Relationship Symbol

Relationship

Relationships are associations between or among entities.

Physical ER diagram symbols

The physical data model is the most granular level of entity-relationship diagrams, and represents the process of adding information to the database. Physical ER models show all table structures, including column name, column data type, column constraints, primary key, foreign key, and relationships between tables.

As shown below, tables are another way of representing entities. The key parts of Entity-relationship Tables are:

Fields

Fields represent the portion of a table that establish the attributes of the entity. Attributes are typically thought of as columns in the database that the ERD models.

Fields Shape

In the image above, InterestRate and LoanAmount are both attributes of the entity that are contained as fields.

Keys

Keys are one way to categorize attributes. ER diagrams help users to model their databases by using various tables that ensure that the database is organized, efficient, and fast. Keys are used to link various tables in a database to each other in the most efficient way possible.

Primary Keys

Primary keys are an attribute or combination of attributes that uniquely identifies one and only one instance of an entity.

Foreign Keys

Foreign keys are created any time an attribute relates to another entity in a one-to-one or one-to-many relationship.

Foreign Key Example

Each car can only be financed by one bank, therefore the primary key BankId from the Bank table is used as the foreign key FinancedBy in the Car table.  This BankID is able to be used as the foreign key for multiple cars.

Types

Types refer to the type of data in the corresponding field in a table. Types can also refer to entity types, which describe the composition of an entity; e.g., a book's entity types are author, title, and published date.

ERD Tables

ER diagram notation

While crow's foot notation is often recognized as the most intuitive style, some use OMT, IDEF, Bachman, or UML notation, according to their preferences. Crow's foot notation, however, has an intuitive graphic format, making it the preferred ERD notation for Lucidchart.

Cardinality and ordinality

Cardinality refers to the maximum number of times an instance in one entity can relate to instances of another entity. Ordinality, on the other hand, is the minimum number of times an instance in one entity can be associated with an instance in the related entity.

Cardinality and ordinality are shown by the styling of a line and its endpoint, according to the chosen notation style.

ERD Notation