[{"query":"a","page":"p1","clicks":10,"date":"x"},{"query":"b","page":"p1","clicks":5,"date":"x"},{"query":"a","page":"p1","clicks":5,"date":"y"},{"query":"c","page":"p2","clicks":2,"date":"y"},]

db.collection.aggregate([{"$group":{"_id":{"page":"$page","query":"$query"},"sum_of_clicks_for_query":{"$sum":"$clicks"}}},{"$project":{"_id":false,"page":"$_id.page","most_clicks_query":"$_id.query","sum_of_clicks_for_query":true}},{$sort:{"sum_of_clicks_for_query":-1}},{$group:{_id:"$page",group:{$first:"$$ROOT"}}},{$replaceRoot:{newRoot:"$group"}}])