Designing your database is one of the most important steps when starting to develop a new application and choosing the right tool for it can make your life much easier. Diagramming your database helps you better understand how your data and entities will be stored and communicate with each other. Many projects suffer in the long run from having overlooked this all-important step, as their application comes to life and mass of data, more and more, any structural change becomes more complicated and time-consuming.
The good news is that there are great tools to help with this task, I have put together a list to help you choose a free diagramming tool.
Also check: 4 free tools to manage MongoDB
Better tools for designing/diagramming a database
dbdiagram.io is an online tool that easily allows for quick prototyping of entities and relationships. I put it in first place as it is my go-to tool on a daily basis when I need to structure a relationship. It has github integration, which allows you to easily version your work.
diagrams.net is an excellent tool for designing your database and the relationship between entities. It has integration with OneDrive and Google Drive to automatically save your projects. As it is a more complete tool, you can in addition to diagram your database, draw flowcharts, organization charts and much more.
Lucidchart follows the same line as diagrams.net, a complete and versatile tool that allows database diagramming, entity relationships, etc. There is a limitation of 100 basic templates in the free plan, but for simple projects this shouldn't be a hindrance.
MySQL Workbench is a tool developed by Oracle focused on administering and building a database in MySQL, however, its diagramming tool is very practical and complete, and can be used to diagram entities and especially SQL databases, making it a powerful offline alternative .