[{"name":"abc","country":"xyz","bloodGroup":"A+","contact":"000001111"},{"name":"abc","country":"xyz","bloodGroup":"A+","contact":"000001111"},{"name":"abc","country":"xyz","bloodGroup":"A-","contact":"000001111"}]

db.collection.aggregate([{"$group":{"_id":{"country":"$country","bloodGroup":"$bloodGroup"},"count":{"$sum":1}}},{"$sort":{"count":-1}}])