db.collection.aggregate()　不会报错，且会和find一样返回所有文档

db.orders.aggregate([
   {$group: {_id: null,count: { $sum: 1 }}}  //全表件数
])

db.orders.aggregate([
   {$group: { _id: null, total: { $sum: "$price" }}}　//全表价格总和
])

db.users.aggregate([
  {$match:{access:"valid"}},
  {$group:{_id:"$cust_id",total:{$sum:"$amount"}}},
  {$project:{_id:1,status:1,amount:1} },
  {$sort:{total:-1}}            // 1:升序  -1:降序
])


// 展开数组
db.students.aggregate({$unwind:"$score"})
{
name:"张三",
score:[
    {subject:"语文",score:84},
    {subject:"数学",score:94},
    {subject:"英语",score:74}]
}

{name:"张三",score: {subject:"语文",score:84}}
{name:"张三",score: {subject:"数学",score:94}}
{name:"张三",score: {subject:"英语",score:74}}


// 展开2层数组
db.students.aggregate(
    {$unwind:"$hobbies"},
    {$unwind:"$hobbies.type"}
)


// $bucket用法
db.products.aggregate({
    $bucket:{
        groupBy:"$price",
        boundaries:[10,20,30,40],
        default:"other",
        output:{"count":{$sum:1}}
    }
})

// $facet用法
db.products.aggregate({
    $facet:{
        price:{
            $bucket:{ }
        },
        year:{
            $bucket:{ }
        }
    }
})