What is an Entity Relationship Diagram (ERD)? - All You Need to Know

What is an Entity Relationship Diagram (ERD)?

An Entity Relationship Diagram (ERD) shows how entities (such as people, objects or concepts etc.) relate to each other in a particular system. Generally, an ERD does not define business processes, but graphically displays business data patterns. In this case, the entity can be regarded as a noun and the relation can be considered as a verb. ERD can help users conceptualize abstract elements in order to discuss and understand the relationship between different concepts. The simple ERD example below shows you the database system of a herbal company. Feel free to click on it to see more.

ERD Background Overview

In fact, the study of the relationship between different objects can be traced back to some ancient Greek philosophers. By the 1970s, data modeling had become very popular, but there was no universal and effective method to standardize it. In 1976, Peter Chen, who taught at Massachusetts Institute of Technology (MIT), first introduced the entity relationship model in his paper and designed corresponding charts for his findings. Peter was inspired by the data structure diagram (Bachmann diagram) proposed by Charles Bachmann in 1960s. Later, Charles Bachman and James Martin made improvements to the basic principles of ERD, which also promoted the development of the Unified Modeling Language (UML) diagram in computer science.

Learn about EER Diagrams and the differences between these 2 diagrams.

ERD Basic Elements

Most ERDs have evolved from the design of physical databases. Don't worry about their complexity, you can easily learn this field by checking out the following three basic ERD elements:

Entities

Entities are usually divided into five categories: concepts, locations, roles, events, and things (people, places, events, sales, customers, company departments, students, or products etc.). An entity is shown in a database table in a data system, and each row of the table represents an instance of the entity. For example, the Name entity in the table below could be Jack, Alice and so on.

The most common subtypes of ERD entities are:

Attributes

Attributes are the detailed information collected for entities (including the characteristics of the data etc.). In a school management system, the attributes of students can be their names, family addresses, classes and contact phone number and so on. The attributes of job hunters can be their age, previous position, and location etc.

Relationships

Relationships describe how entities interact with each other. Besides, relationships are usually marked with verbs, for example, a customer buys software online. In addition, if an entity connects the occurrences between two different entities, then it is called a recursive relationship. For example, a department manager may be the supervisor of his subordinates, or one of the subordinates of his supervisor.

ER Diagram Major Cardinality Classifications

There are three main types of relationships between entities:

One-to-one

An instance of the entity A is associated with another instance in the entity B. For example, an enterprise employee has an unique company ID number.

One-to-many

An instance of the entity A is associated with one or more instances of the entity B, but not vice versa. For example, the marketing department may have many employees, but each marketing department employee usually corresponds to only one department: the marketing department. An order number usually has only one corresponding purchaser, but a purchaser can have more than one order number of the ordered products. Here is another example that shows you a product can have a series of unique entities including the Product No.

Many-to-many

An instance of the entity A is associated with one or more instances of the entity B, and vice versa. For example, a freelancer may offer part-time services to several companies, meanwhile the company may also cooperate with different part-time workers.

Key ERD Symbols

Typical ERDs usually have the following symbols:

Name

Symbols

Descriptions

Entity

Boxes represent entities. Strong entities and weak entities can both be represented by rectangles.

Relationship

Shapes in diamonds represent relationships between entities.

Simple Attribute

In ERD, attributes are shown in the form of circles or ovals and are classified as simple, composite, derived, single-value and multi-value. Simple attributes indicate that attributes cannot be further subdivided, for example, an employee's work number. The composite type is the sub-attribute that spring from an attribute.

Derived Attribute

The derived attribute is calculated or derived from another attribute, such as the length of service from a date of enrollment.

Multi-valued Attribute

The multi-value means more than one attribute is denoted, for example, a number of different ordering ID for a buyer. A single-value attribute can be combined with both single-value attributes and multi-value attributes.

Cardinality Styles

In an ERD, cardinality symbols have different forms and styles. The main types are Bachman, Crow's Foot and Barker as shown on the left.

ER Diagram Data Models

ERD usually has the following presentations: the conceptual data model, the logical data model, and the physical data model. Although these three forms all contain entities, relationships and attributes, they are used by different groups of users. Normally, business analysts prefer to use the conceptual and logical models, while database engineers like using the logical and physical models.

Conceptual Data Model

This ERD model lacks specific details, and it only contains the definition and general tables of the entities in a data system. The conceptual data model provides an overview of your project scope and the general architecture of the system, and show how data sets are related to each other. However, for smaller systems, it may not be necessary to draw such a model.

Logical Data Model

This ERD model is more detailed than the conceptual data model because the former type contains detailed attributes of entities and relationships between different sets. The logical ER model is also developed independently of a specific database management system.

Physical Data Model

The physical data model assigns more detailed technical details to each entity and it offers users with guidance on how to construct their systems and associated data. Physical data models are usually based on logical data models.

ERD Applications

Since ERDs are easy to understand even for non-professional or non-technical users, they are widely used in the following fields:

Here is an ERD example that shows a customer order system of a trade enterprise. You can click on it to free download its PDF version.

Limitations of ERDs

The limitations of ERD mainly including the following aspects:

Relationships among ERD, DFD and BPD

In practice, ERDs can also be used in combination with other related database diagram types such as:

More Free ERD Templates and Examples

Free download these ERD templates below for your projects. Click on any of their names to see more details.

Faculty Info ER Diagram Template	Storage ER Diagram Template	Pet Store ER Diagram Template

College System Template	Course System Template	Blended Model ER Diagram Template

How to Create an ER Diagram? - a Simple but Widely Used Guide

Drawing an ER diagram is not complex if you do the following easy steps:

Step 1: Define your Purpose and Scope

Do you need an ERD for your business process, or to develop new software databases? Depending on the actual needs, as mentioned earlier, you may need to draw a conceptual, logical, or physical model.

Step 2: Add Entities

Add and name your entities correctly. You should also check out whether the tables in your ERD are sufficient to store enough data. Add or subtract your entity tables accordingly.

Step 3: Decide and Insert Your Attributes

In this step, you should decide your entity attributes and give meaningful and understandable attribute names.

Step 4: Determine the Specific Relationships

Now, you should determine and label the specific relationship type for each of your connections (one-to-one, one-to-many, many-to-many). Don't worry about isolated entities (though this is not common).

Step 5: Review your ERD

Reduce data redundancy by restructuring entities, and check for missing important entity details. You can also split complex data into separate entities.

Further Reminders of Creating an ER Diagram

How to Draw an ERD with Software?

There is one free ERD software that can satisfy your needs and help you create ER diagrams quickly with ease.

The process is even easier than you thought. Just try the following quick steps.

Step 1 - Start the ERD Software

Start EdrawMax and go to "Available Templates" - "Database", double-click to open a new drawing page. Alternatively, you can select one of the preset templates for your work.

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Step 2 - Add Editable Elements

To get the symbols, simply click Library button on the top left corner under the Ribbon, and navigate ERD and database symbol category. Next, drag and drop the built-in ERD shapes from the left-hand library to the right-hand canvas for further customizations, such as changing sizes/colors, or group a set of your individual entity shapes.

Step 3 - Edit Text

Double click on the default text to edit your own words.

Step 4 - Connect Your Entities

Connect your individual entities by inserting straight connectors. Alternatively, you can drag and drop different styles of ERD connectors from the left-hand preset libraries.

Step 5 - Apply a Theme to Your ER Diagram

It's easy to change the whole ER diagram's theme, shadow effects and background based on the built-in auto-create tools.

Step 6 - Save/Print/Share/Export

Once done your work, you can save it on the Edraw team or personal cloud, print it in different paper sizes including A3 and A4, one-click share it on your social networking platforms, or export it to a wide range of different formats such as JPG, PDF, MS Word and more.

Drawing Your Own ER Diagrams with EdrawMax

Now, you have learned all the key aspects of the ERD. It's your turn to simply create your ER Diagrams with the cross-platform EdrawMax! Conveniently craft visually engaging diagrams by using the preset standard ERD symbols and auto-create tools. Feel free to drag and drop built-in shapes, quickly connect your entity boxes, change the default diagram theme and do more with the software. Save and share your files on the Edraw team or personal cloud for your collaboration. Have a try right now by clicking on the Download PC Version or Mac Version button at the end of this page!