I have free and paid themes tables.
Free themes have download link
Paid themes have buy link and price.

Here is a database schema in graphical format:

[!["theme" can be either "paid theme" or "free theme" with different fields](https://i.stack.imgur.com/IoMKq.png)](https://i.stack.imgur.com/IoMKq.png)

A theme cannot be both paid and free. How can I prevent both tables (FreeThemes, PaidThemes) from having data for a theme at the same time?