I learned SQL working on a "database as a product". The database was filled with medical ontologies. It was a perfect environment for learning. We were always l

I learned SQL working on a "database as a product". The database was filled with medical ontologies. It was a perfect environment for learning. We were always looking for obscure things in that database, and rarely changed data, just selects for days. In the end I once wrote a query spanning about 100 lines that used common table expressions and found it quite maintainable. Where can an SQL beginner find such a database to experiment with? I was fortunate because my job provided me this database to experiment with, but what about those who are not so fortunate?