[{score:1,},{score:2,},{score:3,},{score:4,},{score:5,},{score:6,},{score:7,},{score:8,},{score:9,},{score:10,},{score:1,value:10},{score:3,value:10},{score:1,value:10},{score:4,value:10},{score:1,value:10},{score:5,value:10},{score:5,value:10},{score:10,value:10},]

db.collection.aggregate([{$bucket:{groupBy:"$score",boundaries:[0,1,2,3,4,5,6,7,8,9,10,11],output:{avg:{$avg:"$value"}}}},{$addFields:{avg:{$ifNull:["$avg",0]}}}])