From Mongo to SQL?

There are many resources to help people who know SQL to understand mongo… usually through different metaphors (tables = collections) etc.

Are there any good resources to go form mongo to SQL? I’m one of the minority of developers who started with noSQL.

I’m a little concerned about the rigidity I keep hearing about SQL, and how it is harder to iterate on your models… where mongo it’s not a huge deal if you don’t know what you need.

Maybe some (free) beginner books people could suggest?