aggregate.js

// RDB - select birth , working , count(*) from users group by birth , working

// mongoDB aggregation
 
db.users.aggregate([
   { $project : {   a : "$birth" 
                ,   b : "$working" 
                , _id : 0 }}
  ,{ $group : { _id : { a:"$a",b:"$b"} 
              ,    n: { $sum:1       }
              }
    }
 ])