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Entity Relationship Diagrams explained by Sonic the Hedgehog

Helen Anderson on May 20, 2019

The Entity-relationship diagram (ERD) shows the relationships between tables and the common keys that connect them. Initially, they may appear to b...

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Leslie Tang • May 20 '19 • 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:

visualization of relationships between data
serve as plans for implementing the database
communicating these plans
determine requirements
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.

Helen Anderson • May 21 '19

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:

Changelog: Create Series of Posts

Ben Halpern ・ 2 min read

#changelog #meta

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

How to Manage the TOC of Articles on dev.to?

🇫🇷 Boris Jamot ・ 0 min read

#help #meta #markdown #toc

Leslie Tang • May 21 '19

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

Helen Anderson • May 21 '19

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

Youth • May 21 '19

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;
    }
  }
};

Thomas Brok • May 21 '19 • 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 ;-).

Helen Anderson • May 22 '19

Nice catch, now edited :)

Didin 👨‍💻 • Nov 8 '19

Nice post!!!
I think you can try dbdesigner.id 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 dev.to/didin1453fatih/database-des...

Alex Antra • May 20 '19

Gotta go fast! Great explanation!

Helen Anderson • May 21 '19

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

Pedro • Oct 15

Hi. Wich is the diference between "one" and "one and only one". When i started in this world, 2002, one meaning "only one". No put two marks min and max because the value was the same, to min to max. I can see lucidchart and a lot of people, now use 2 marks, but then, i dont understand if 2 diferent values or the same value but diferent notation. Thanks for your help.

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Ted • May 23 '19

very basic stuff

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