[{"_id":ObjectId("602252684a43d5b364f3e6ca"),"student_id":1234,"study_num":8,"finish_date":"20210209",},{"_id":ObjectId("602257594a43d5b364f4cc6a"),"student_id":1234,"study_num":7,"finish_date":"20210209",},{"_id":ObjectId("5fbb65580d685b17fa56e18f"),"student_id":2247,"study_num":6,"finish_date":"20210209",}]

db.collection.aggregate([{"$match":{"student_id":1234,}},{"$group":{"_id":"$finish_date","study_sum":{$sum:"$study_num"}}},{"$group":{"_id":null,"study_sum":{$sum:"$study_sum"},count:{$sum:1}}}])