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Entity Relationship Diagrams explained by Sonic the Hedgehog

Helen Anderson
Data wrangler at Kiwibank, AWS Data Hero, and tag moderator on
Originally published at Updated on ・2 min read

The Entity-relationship diagram (ERD) shows the relationships between tables and the common keys that connect them. Initially, they may appear to be a jumble of spaghetti noodles.

In this example, I'll use data from my favorite Sega Master System game, Sonic the Hedgehog, and illustrate how the zones, acts, and boss levels work together.

Conceptual Data Model
Logical Data Model
Physical Data Model
What do the circles and arrows mean?
Which systems can we model?
How do we start?

Conceptual data model

Level one!

The conceptual data model. This is a high-level overview of the different entities in the model and their relationships.


In our example, we show that in Sonic the Hedgehog 1, zones can have many acts, these levels may have special stages, may have bosses, but there is only one boss per zone.

This model features entities, the rectangles. This is a physical thing, a fact or an event. The relationships between these entities are shown with diamonds.

Logical data model

The logical data model adds more details to our model. In this diagram, we add the relationships between the tables using crows foot notation (more on that in a little bit) and add primary and foreign keys to show exactly how they are related.


Physical data model

And finally the physical data model shows how the model will actually be implemented. This features the data types and names of the tables in our database. In this case, all the tables record attributes so are dimensions.


What do the circles and arrows mean?

This is called crows foot notation and shows the relationships by using a 'crows foot', line or circle on either side of the line connecting the entities.


Which systems can we model?

Structured data in relational systems. We need to be able to see relationships in the form of tables, columns and keys.

How do we start?

Lucidchart does a great job of making it easy to map out your diagram when you are ready to publish, but there are many tools out there to help you get started.

The very best way to get started is with a low tech option. Get out a pen and a big piece of paper. Make this your first step before you open the tools. Plus it will get you away from your screen for a little bit. Good luck!

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Discussion (11)

tangweejieleslie profile image
Leslie Tang • Edited

Hi Helen, that's a nice post! I've clicked through this post because of Sonic but left with some newfound knowledge. 😁

I just want to check if I got what I'm reading right...

In short, some uses of Entity Relationship Diagrams(ERD) are:

  1. visualization of relationships between data
  2. serve as plans for implementing the database
  3. communicating these plans
  4. determine requirements
  5. debug system

And those uses translate naturally into "Why we use ERD".

While I can clearly see how ERD is used to serve the first 3 purposes, I have no more than a vague idea on using them for 4 and 5. If possible, do you mind elaborating on using ERD to determine requirements (and what sort of requirements) as well as using them to debug systems.

Thank you so much for this wonderful post! I will probably be reading through the rest of the BI series as well 😄

P.S. I have another question that's not really related to the content of the article. The part where you add the 5 circles to show the article being part of the "Business Intelligence Series" is so cool! I just wanted to know what is it called so I can also add it to my own posts.

helenanders26 profile image
Helen Anderson Author

Hi Leslie,

Thanks for the nice comments, I really enjoyed writing it.

Regarding requirements gathering, what I was trying to get across was that its part of the process we go through with stakeholders.

When building a new model there's lots of back and forth and clarification of what's required now, later and what can be delivered after go live. Ideally all the requirements are locked down early but in reality, this doesn't always happen.

At the first step, with the Conceptual Data Model, we can go back to the stakeholders to double check the requirements and do our best to avoid scope creep. Hopefully ;)

As far as debugging goes the ERD gives us a point of reference for how things 'should' work. If multiple team members are releasing changes to the model we can visually see how it fits together and where the dependencies are.

It's also helpful when doing any kind of database migration.

Excellent questions!

I really like stringing my posts together in series and use the instructions here to do so:

I'm also a fan of the Table of Contents at the top of my posts:

tangweejieleslie profile image
Leslie Tang

Thanks for replying Helen, your explanations are really clear!😄
Also, thank you for pointing me to these articles.

Thread Thread
helenanders26 profile image
Helen Anderson Author

You're welcome, looking forward to reading more of your posts :D

qcgm1978 profile image

I love what you describe and your words. The following is a code demo by js:

// Conceptual Data Model
const EntityRelationshipDiagrams = {
  Zones: class {
    // Logical Data Model
    constructor() {
      this.ZoneId = EntityRelationshipDiagrams.PhysicalDataModel.getId(105);
      this.voice = "truths";
    // Physical Data Model
    speak() {
      return `${this.ZoneId} ${this.voice} as work`;
  Boss: class {
    // Logical Data Model
    constructor() {
      this.BossId = EntityRelationshipDiagrams.PhysicalDataModel.getId(0);
      this.voice = "barks";
    // Physical Data Model
    speak() {
      return `${this.BossId} ${this.voice} in the meeting`;
  // external Physical Data Model
  PhysicalDataModel: {
    getId(num) {
      return num < 100
        ? this.getRandomInt(0, 100)
        : this.getRandomInt(num, 1000);
    getRandomInt(min, max) {
      min = Math.ceil(min);
      max = Math.floor(max);
      return Math.floor(Math.random() * (max - min + 1)) + min;
tphbrok profile image
Thomas Brok • Edited

Nice article!

Just noticed that the entity 'Zones' has an attribute 'ActID', which shouldn't be there if I am correct. A one-to-many relationship should only have a foreign key on the 'many'-side, as having one on the other side does not make sense. Having 'BossID' there would make more sense ;-).

helenanders26 profile image
Helen Anderson Author

Nice catch, now edited :)

didin1453fatih profile image
Didin 👨‍💻

Nice post!!!
I think you can try to create Entity Relationship Diagram 😎😎😎.
This can help you to collaborate with your team on web and cut off your step in generate SQL Create database.
I write this article to explain more about this devtool

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ronsoak profile image

Gotta go fast! Great explanation!

helenanders26 profile image
Helen Anderson Author

Love it! I should have worked that into the post somewhere

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