[{"_id":101,"details":[{"run_date":"2021-04-18","ap_name":"EVPN","sub_ap_name":"EVPN TOR"},{"run_date":"2021-04-18","ap_name":"EVPN","sub_ap_name":"EVPN Mobility"},{"run_date":"2021-04-17","ap_name":"Multicast","sub_ap_name":"Multicast Layer 2"}]},{"_id":102,"details":[{"run_date":"2021-04-17","ap_name":"Multicast","sub_ap_name":"Multicast Layer 2"}]},{"_id":103,"details":[{"run_date":"2021-04-17","ap_name":"EVPN","sub_ap_name":"EVPN Layer 2"}]}]

db.collection.aggregate([{$match:{"details.ap_name":"EVPN"}},{$unwind:"$details"},{$match:{"details.ap_name":"EVPN"}},{$group:{_id:"$details.ap_name","sub_ap_names":{$addToSet:"$details.sub_ap_name"}}}])