I thought you might find useful this PDF of a translation from SQL to MongoDB's proprietary query/aggregation framework.
Translate SQL to MongoDB MapReduce
I keep hearing people complaining that MapReduce is not as easy as SQL. But there are others saying SQL is not easy to grok. I’ll keep myself away from this possible flame war and just point you out to this ☞ SQL to MongoDB translation PDF put together by Rick Osborne and also his ☞ post providing some more details.