Database
[{"customerID":1,"acctDetails":[{"accType":"Saving","balance":100},{"accType":"checking","balance":500}]},{"customerID":2,"acctDetails":[{"accType":"Saving","balance":500}]}]
Query
db.collection.aggregate([{"$unwind":"$acctDetails"},{"$group":{"_id":"$acctDetails.accType","totalBalance":{"$sum":"$acctDetails.balance"}}}])