Some relational databases like Postgres and MySQL support JSON columns. This is useful if, for example, you want to create an ecommerce application with multiple product variations stored in a single table, but with different fields - that is called Single Table Inheritance (STI). You can have some regular columns for the common fields, and a JSON column for the specific ones.
PostgreSQL JSON can be indexed and searched. They also have a JSONB format which is more efficient. Though I believe Mysql's JSON operators are more standards compliant.
If your app is built with Rails, you can use this library to help you on that (I'm not affiliated with it): https://github.com/jrochkind/attr_json