Database
[{"weight":70,"height":180},{"weight":80,"height":179},{"weight":100,"height":169}]
Query
db.collection.aggregate([{$group:{_id:1,avgBmi:{$avg:{$divide:[{$toDouble:"$weight"},{$pow:[{$divide:[{$toDouble:"$height"},100]},2]}]}},minBmi:{$min:{$divide:[{$toDouble:"$weight"},{$pow:[{$divide:[{$toDouble:"$height"},100]},2]}]}},maxBmi:{$max:{$divide:[{$toDouble:"$weight"},{$pow:[{$divide:[{$toDouble:"$height"},100]},2]}]}}}}])