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Barker’s Notation

When looking at different kinds of ERD notations, it is hard not to come across Barker’s ERD notation, which is commonly used to describe data for Oracle. Richard Barker and his coworkers developed this ERD notation while working at the British consulting firm CACI around 1981, and when Barker joined Oracle, his notation was adopted.

Let’s take a closer look at Barker’s syntax.

The most important components in the ERD diagram are:

  • entities, which can be thought as physical objects or elements that can be uniquely identified, and
  • relationships, which capture how entities are related to one another.

Entities

When drawing elements in the Barker notation, some rules should be followed.

  • Entity – is represented by a rounded corner rectangle. The entity name should be in the upper part of rectangle and in singular form.

    Barker’s ERD notation – a sample entity

  • Attributes – describe the characteristics of a particular entity instance. An attribute can be of three types:

    1. Unique Identifier – uniquely identifies an entity instance
    2. Mandatory – its value cannot be null
    3. Optional – its value can be null

    as presented in the diagram below.

    Barker’s ERD notation – a sample entity with attributes

Relationships

A relationship links two or more entity instances together. A relationship is commonly represented by a straight line.

  • Optionality of a relationship

    A mandatory relationship is represented by a straight line, which specifies that each instance of an entity must be related to another instance.

    Barker’s ERD notation – mandatory relationship

    An optional relationship is represented by a dashed line, which specifies that each instance of an entity may be related to another instance.

    Barker’s ERD notation – optional relationship

    It’s important to note that only binary relationships are allowed in a Barker notation.

  • Degree of relationships:

    one-to-one – each entity instance is related to just one entity instance.

    Barker’s ERD notation – one-to-one relationship

    one-to-many – each entity instance is related to multiple entity instances.

    Barker’s ERD notation – one-to-many relationship

    many-to-many – multiple entity instances are related to multiple entity instances

    Barker’s ERD notation – many-to-many relationship

    A relationship is always made up of two perspectives using the following notation:

    Barker’s ERD notation – a relationship made up of two perspectives

    One or more employees may work in each department; each employeer must work in one department.

  • UID bar

    A bar “|” across one end of a relationship line indicates that the relationship is a component of the primary identifier for the entity type at that end.

    Barker’s ERD notation – one-to-many relationship with UID bar

    Each individual UID Bar in the ORDER ITEM represents the fact that the foreign key represented by the relationship is also a primary key of the ORDER entity.

    Barker’s ERD notation – a relationship with UID bars

  • Non-Transferability of Relationships

    In certain situations, once a relationship is set it can never change. For example, BOOK and CHAPTER in the example below. Chapter can’t be moved to a different book. We call this the non-transferability of a relationship and it is represented by a rhombus.

    Barker’s ERD notation – non-transferability of a relationships

  • Subtypes

    There are situations where different entities can be grouped together under one common entity called a super type entity. The nested entities would be the subtype entities.

    Barker’s ERD notation – Super type entity with subtype entities

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