Database
[{"account_id":11,"event":ISODate("2021-05-03T10:33:18.220Z")},{"account_id":11,"event":ISODate("2021-05-03T10:33:19.220Z")},{"account_id":11,"event":null},{"account_id":99,"event":ISODate("2021-05-03T10:33:20.220Z")},{"account_id":99,"event":null}]
Query
db.collection.aggregate([{$match:{account_id:11}},{$group:{_id:null,total:{$sum:1},with_event:{$sum:{$cond:[{$eq:[{$type:"$event"},"date"]},1,0]}}}}])