Creating an Erntity-relationship diagram (ERD).

According to this prompt on Linked In, this Monday Morning I’ve been selected to write an article on creating an Entity-relationship diagram (ERD). OK – lets dust off the college text book and go through the steps to model the relationships between entities in a database.  Linked In has a 750 char limit, and only let me post three times about ERD's breaking this up, so - I moved the whole 3000 character doc out here to my blog!

Generally speaking – and there’s many database specific variants, you will need to Identify Entities. Start by identifying all the entities within the system you're modeling. Things. Nouns. (Person, Place, Paint Color, Family Size, etc. Entities are typically nouns representing objects, concepts, or things about which data is stored.

Next, you need to review each noun (entity) and Define the Attributes. Every entity is going to have descriptive characteristics that identify and define its attributes. You will represent these attributes as columns in a database table – a list of colors, regions or states in a country.

Now, with your entities and attributes, you Determine Relationships. Identify how the entity relates to it’s attribute. Can a car have one color? Or many colors? Relationships define how entities interact with each other. You can look at these as verbs – has, lives, is, contains, etc.

A bit more specificity to your verbs (relationships) – what is the cardinality of each relationship. Cardinality describes the relationships between entities, such as one-to-one, one-to-many, or many-to-many. As above – can a car have one or many colors? Would a person live in one, or many states. Would many cars have the same color?

Ok – thinking part is over; now we draw! Using tools specific to your database, or one of many online or paid tools, Draw the ERD. There are standard notations to use like Crow's Foot notation and Chen notation. Choose one that suits your preference or organizational standards. Specific directions will vary by tool, but the general steps are to:

1. Create Entity Boxes: Represent each entity as a box in the diagram. Label the box with the entity name and list its attributes inside the box.
2. Add Relationship Lines: Draw lines between related entities to represent the relationships. Label each line with the appropriate relationship type and cardinality.
3. Refine the Diagram: Review the diagram to ensure it accurately represents the relationships and attributes of the entities. Make adjustments as necessary to clarify the structure.
4. Document the ERD: Document the diagram to provide additional context and explanation. Include a legend that explains the notation used, as well as any assumptions made during the modeling process.

And the most important part – none of this takes place in a vacuum. Review and Validate the ERD with stakeholders, subject matter experts, or other team members to validate its accuracy and completeness. Make revisions based on feedback as needed.

There you go, LinkedIN and followers – a basic document on creating Entity Relationship Diagrams. Enjoy!

Will England