• select * from db.co order by id DESC limit 5

 db.co.find().limit(5).sort({"_id":-1})

• find some items not exists in collection:

    db.co.find({ "item":
            { "$exists" : False }
        })

• skip

db.co.find().skip(5).limit(5)

• delete from db.co where age < 30

db.co.remove({ 'age' : {$lt : 30}})

• select age from db.co

db.co.find({},{"age" : 1})

• select others(not age) from db.co

db.co.find({},{"age" : 0})