I drew my first Entity Relationship Diagram (ERD) over 20 years ago. Used some drawing package to make the pictures. Not a whole lot has changed since then. The ERD was invented by Peter Chen back in 1976. It looks a lot like a flowchart. The ERD repsents an information system. It shows the logical structure of a database.
The ERD deals with entities and relationships. It does not show a specific entity. Instead it deals with entity sets. Drawing up an ERD is part of a data modeling technique. In the end, it is used to create a relational database. An ERD is not well suited for unstructured data.
There are different style to draw the ERD:
- Information engineering
- Peter Chen
- Charles Bachman
- James Martin
Of course there are tons of tools that can help you draw an ERD. Here are some commercial ones I have heard about:
- ERwin
- MagicDraw
- Oracle Designer
- Rational Rose
- System Architect
- TOAD Data Modeler
I mentioned before that ERDs have entities and their cardinality. Entities are the objects that have data. Cardinality is the relationship between these objects in terms of numbers. The main types of such relationships are:
- One-to-one
- One-to-many
- Many-to-many
