Coda is an online graphical tool for creating relational databases (think: Spreadsheets on steroids). (Link)
Pricing
Coda has a limited free version but requires a premium subscription for it to be sufficient for most uses. Students and educators get free individual access. Coda's pricing scheme for teams is rather convoluted so if you are looking to use it for collaboration, expect to do some research first.
Alternatives
  • Obsidian
  • Notion is an online notetaking database app. It has many of the same features for creating relational database, but I moved away from it because it had some annoying bugs that were not fixed for several years. Notion is somewhat better suited for creating a personal knowledge base than Coda, but I would recommend Obsidian over both.

Coda is a powerful online tool for creating relational databases. I have used Coda to create an extensive to-do list app that is customized with all sorts of bells-and-whistles that were unavailable in other apps and would have taken many times longer to code from scratch outside of Coda.