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2012
Ultimate Guide to ER Diagrams
An Entity Relationship Diagram (ERD) is a visual representation of different data using conventions that describe how these data are related to each other. For example, the elements writer, novel, and consumer may be described using ER diagrams this way:
ER diagram with basic objects
In the diagram, the elements inside rectangles are called entities while the items inside diamonds denote the relationships between entities. This ER diagram tutorial for beginners covers most things related to ER diagram, for quick navigation use the links below.
- ER Diagram Usage
- History of ER diagrams
- ER Diagrams Symbols and Notations
- How to Draw ER Diagrams
- ER Diagram Templates
- Benefits of ER Diagrams
ER Diagrams Usage
While able to describe just about any system, ER diagrams are most often associated with complex databases that are used in software engineering and IT networks. In particular, ER diagrams are frequently used during the design stage of a development process in order to identify different system elements and their relationships with each other. For example, an inventory software used in a retail shop will have a database that monitors elements such as purchases, item, item type, item source and item price. Rendering this information through an ER diagram would be something like this:
ER diagram example with entity having attributes
In the diagram, the information inside the oval shapes are attributes of a particular entity.
History of ER Diagrams
ER diagrams are visual tools that are used in the Entity-Relationship model initially proposed by Peter Chen in 1976 to create a uniform convention that considers both relational database and network views. Chen envisioned the ER model as a conceptual modeling approach that views real world data as systems of entities and relationships. Entities are data objects that maintain different relationships with each other. Additionally, entities are also described further using attributes.
Since 1976, the ER model has been expanded and is sometimes used in business management, product development, and strategy formulations. However, database design remains its primary application.
ER Diagram Symbols and Notations
Elements in ER diagrams
There are three basic elements in an ER Diagram: entity, attribute, relationship. There are more elements which are based on the main elements. They are weak entity, multivalued attribute, derived attribute, weak relationship and recursive relationship. Cardinality and ordinality are two other notations used in ER diagrams to further define relationships.
Entity
An entity can be a person, place, event, or object that is relevant to a given system. For example, a school system may include students, teachers, major courses, subjects, fees, and other items. Entities are represented in ER diagrams by a rectangle and named using singular nouns.
Weak Entity
A weak entity is an entity that depends on the existence of another entity. In more technical terms it can defined as an entity that cannot be identified by its own attributes. It uses a foreign key combined with its attributed to form the primary key. An entity like order item is a good example for this. The order item will be meaningless without an order so it depends on the existence of order.
Weak Entity Example in ER diagrams
Attribute
An attribute is a property, trait, or characteristic of an entity, relationship, or another attribute. For example, the attribute Inventory Item Name is an attribute of the entity Inventory Item. An entity can have as many attributes as necessary. Meanwhile, attributes can also have their own specific attributes. For example, the attribute “customer address” can have the attributes number, street, city, and state. These are called composite attributes. Note that some top level ER diagrams do not show attributes for the sake of simplicity. In those that do, however, attributes are represented by oval shapes.
Attributes in ER diagrams, note that an attribute can have its own attributes ( composite attribute )
Multivalued Attribute
If an attribute can have more than one value it is called an multivalued attribute. It is important to note that this is different to an attribute having its own attributes. For example a teacher entity can have multiple subject values.
Example of a multivalued attribute
Derived Attribute
An attribute based on another attribute. This is found rarely in ER diagrams. For example for a circle the area can be derived from the radius.
Derived Attribute in ER diagrams
Relationship
A relationship describes how entities interact. For example, the entity “carpenter” may be related to the entity “table” by the relationship “builds” or “makes”. Relationships are represented by diamond shapes and are labeled using verbs.
Using Relationships in Entity Relationship Diagrams
Recursive Relationship
If the same entity participates more than once in a relationship it is known as a recursive relationship. In the below example an employee can be a supervisor and be supervised, so there is a recursive relationship.
Example of a recursive relationship in ER diagrams
Cardinality and Ordinality
These two further defines relationships between entities by placing the relationship in the context of numbers. In an email system, for example, one account can have multiple contacts. The relationship in this case follows a “one to many” model. There are number of notations used to present cardinality in ER diagrams. Chen, UML, Crow’s foot, Bachman are some of the popular notations. Creately supports Chen, UML and Crow’s foot notations.The following example uses UML to show cardinality.
Cardinality in ER diagrams using UML notation
Tips on How to Draw ER Diagrams
Because ER diagrams are simple enough to understand, just about anyone can create them. However, two different ER diagrams describing the same system may still be radically different in terms of their simplicity, completeness, and efficiency at communicating the system. In other words, there are good ER diagrams and there are poor ones.
Because this ER tutorial focuses on beginners below are some tips that will help you build effective ER diagrams:
- Identify all the relevant entities in a given system and determine the relationships among these entities.
- An entity should appear only once in a particular diagram.
- Provide a precise and appropriate name for each entity, attribute, and relationship in the diagram. Terms that are simple and familiar always beats vague, technical-sounding words. In naming entities, remember to use singular nouns. However, adjectives may be used to distinguish entities belonging to the same class (part-time employee and full time employee, for example). Meanwhile attribute names must be meaningful, unique, system-independent, and easily understandable.
- Remove vague, redundant or unnecessary relationships between entities.
- Never connect a relationship to another relationship.
- Make effective use of colors. You can use colors to classify similar entities or to highlight key areas in your diagrams.
You can draw entity relationship diagrams manually, especially when you are just informally showing simple systems to your peers. However, for more complex systems and for external audiences, you need diagramming software such as Creately’s to craft visually engaging and precise ER diagrams. The ER diagram software offered by Creately as an online service is pretty easy to use and is a lot more affordable than purchasing licensed software. It is also perfectly suited for development teams because of its strong support for collaboration.
ER Diagram Templates
Below are some ER diagram templates so you can get started quickly. Clicking on the image and in the new page that opens click the “Use as Template” button.
ER Diagram Template of exam database ( Click on the image to use as template )
A basic ER diagram template for a quick start
Basic ER Diagram template ( Click to use as template )
Benefits of ER diagrams
ER diagrams constitute a very useful framework for creating and manipulating databases. First, ER diagrams are easy to understand and do not require a person to undergo extensive training to be able to work with it efficiently and accurately. This means that designers can use ER diagrams to easily communicate with developers, customers, and end users, regardless of their IT proficiency. Second, ER diagrams are readily translatable into relational tables which can be used to quickly build databases. In addition, ER diagrams can directly be used by database developers as the blueprint for implementing data in specific software applications. Lastly, ER diagrams may be applied in other contexts such as describing the different relationships and operations within an organization.
References
1. Entity-relationship model as published on Wikipedia.
2. Entity Relationship Diagram by Mike Chapple as published on the About.com website
3. Entity-Relationship Modelling by Craig Borysowich as published on the Toolbox.com website
About the Author: Nishadha Silva
Software engineer turned tech evangelist. I handle marketing stuff here at Creately including writing blog posts and handling social media accounts. In my spare time I love to read and travel. I write about marketing stuff in my digital marketing blog. Since you are here make sure to follow @Creately for awesome online diagramming tips.
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About Creately
Creately is an easy to use Online Diagramming software - purpose built for team collaboration. Powerful features and an intuitive interface make Creately ideal for teams working together on everything from Business Diagrams, Strategy Maps, Flowcharts, Web Mockups to UML designs.
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best sites for erd diagram. can any one tell me the date of the artical posted??
The model that you espouse (attributes in ovoids, aka ‘the lollipop model’) is OK for small systems with very limited number of attributes per entity. Unfortunately, some students try to use it for llarger assignments (20 entities with up to 20 attributes per entity) and the whole model becomes unreadable unless they use A2-sized paper. If I never see another model of this type, it will too soon!!
A very good explanation for ER diagrams.
Information in this site are very usefull.tankx v m.
I am an Engineering Student and needed this for my exams.
I can now surely bet on it that I can crack any ER-Diagram
Thanks to author.
i need a ISA relationship with some example
Jus wrote my Database exam, wish id seen this much earlier.
Ya its very good explanation about ER-Diagrams
very nice simple and attractive explanations.
i cleared much of my concept and doubts…….thanks
thanx a Lot….
this is very useful for us. so easy to understand.
Its interesting… Nice presentation…. Thnx
Very to the point and clear explanation. Good read to refresh the concepts just in few minutes. Thanks.
A lucid and elaborate explanation. Appreciable.
gud explanation…
Thank u ,this was very big help for us….
best example of ERD
A very clear explanation of ERD with the aid of diagrams.thanx.
superb examples of ERD
Hi, I have to do an ER model for a travel game app but we have not been shown how in college. Can anyone please give me some ideas on how to go about this.
The player or multiple players are required to navigate their way to the objective location using the transport infrastructure in a city or area, the games A.I places them in. The maps will be updated on a frequent basis to add variety to the user’s experience.
Thanks
i want a relationship between er diagram and uml diagram
vry clear explanation
cleared my most of the doubts , thank you
Thanks for sharing , it is very useful for database designing.
Thank you sir ,I don’t know how to create a ERD, but now I clear this.
Very useful
Explanation is very clear
actually,this is good stuff…thanks lots…
THIS IS VERY RELEVANT FOR MY SAAD PAPER EXAM
thanks a lot..nicely written…
the best website..!
Draw ER diagram for the following “A
teacher can teach many courses. A student
can enrol in many courses. A course may
be a part of one or many programmes. A
teacher can be mentor of many students,
however a student can have only one
mentor.”
Can you answer above Q Please
Examples are very simple and useful to understand ER Diagram.But if you take a simpl eproblem statement and explain how to identify entity,relationships etc… it would be more helpful to understand.
what exactly connector image in entity relationship diagram????
hey, brow.. nice job.. thanks for this.. it gave us so much help.. thanks brow..
I HAVE GOTE A GOOD HELP.THANK U!
thanks,very good explanation for all.
Its really good. Understanding the basic concepts of ER diagram with simple language with very good examples. It is easy to for us to know the contents. Thank You Sir.
i have some five problem like this can u help?
1.Cargo Tracking Management System
2.Supermarket Sales and Inventory Control System
3.Emergency Call Management System
4.Job Management System
I want to what are the entities of these problems?
very nice explanation. Thank you for the time.
Simple, clear, easy.
Thank you so much, that was really helpful.
Thanks…..really good for beginners. it is very good if you can add some solved question regarding ER diagrams…..
Hi Lahiur,
We’ll try to add some example questions in the future.
I have still confusion on creating a relationship between two entities,does relationship block have also attributes of its own please help me with this.
Hi Sachin,
Relationships can have their own attributes as well.
Nice…… thank u………..
simply superb n easy…..
thank u….
thanks dude very usefull for me…….
nice,and very helpful for our subejct in database management system……….
Good Learning Web Site
its an amazing ans ,and i am so satisfied with this ans
I was just wondering…I have an application form with me…the link is attached below. Could you please tell me what entities exist within this application form?
For instance, one entity would be applicant—> attributes consisting of Fname, Lname, address…etc
Your help would be greatly appreciated.
Application form:
http://dairyqueenwest.com/employmentapplication.pdf
you’re a genius,i cherished you.
Its soo hard and I am thinking deeply in my project Callisto Groceru Store. I dont know what is the flow of this grocery store
nicelly defined , i want to make ER diagram for press club. I have some confusions but
after visit this website i have believe Insha Allah i will make easilly.
Very Nice Answer and m really satisfied from it and also my concept is clear from it .
Cool post!
Check out my blog too
nice… Insha allah it may help me in designing ERD for my project…
best defination and example
exact requests have been provided best arrangement of databases
Thanks. My concept is clear from it.
with a diagram can be seen apparently from one component to the other components of related this way will be easier for people to understand the meaning
Im new to dbms….i got help from ur article,,,,thanku so much
Thanks for such a nice explanation\
hello
what we can show relationship of PK and FK using ER Diagram..???