It's not hard, but it's something no-sql databases aren't terribly good at, making it a differentiator for MongoDB. In SQL everything ends up being an exercise

It's not hard, but it's something no-sql databases aren't terribly good at, making it a differentiator for MongoDB. In SQL everything ends up being an exercise in joins, in No-SQL everything ends up being an exercise in map-reduce. The ability to create indexes, like you can easily do in SQL databases, is a handy feature when you have relational or quasi-relational data.