Aggregate Functions

(1) Sum

db.users.aggregate({
$group : {
_id : "$eyeColor",
total : {$sum : 1}
}
})



(2)
db.users.aggregate({
$group : {
_id : "$gender",
total : {$sum : 1}
}
})


(3) Avg

db.users.aggregate({
$group : {
_id : "$gender",
avgAge : {$avg : "$age"}
}
})

(4) max/min
db.users.aggregate({
$group : {
_id : "$gender",
Richest : {$max : "$balance"}
}
})