MongoDB를 사용하는 다단계 $group

다단계 $group을 구현하려면 MongoDB 집계를 사용하십시오. 문서로 컬렉션을 만들자 −

> db.demo76.insertOne({ Name:"Chris","Age" : 21, "CountryName" : 'US' });
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e2bd3e571bf0181ecc42281")
}
> db.demo76.insertOne({ Name:"Chris","Age" : 21, "CountryName" : 'US' });
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e2bd3e571bf0181ecc42282")
}
> db.demo76.insertOne({ Name:"Chris","Age" : 23, "CountryName" : 'UK' });
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e2bd3e571bf0181ecc42283")
}
> db.demo76.insertOne({ Name:"Chris","Age" : 23, "CountryName" : 'UK' });
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e2bd3e571bf0181ecc42284")
}
> db.demo76.insertOne({ Name:"Chris","Age" : 21, "CountryName" : 'AUS' });
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e2bd3e571bf0181ecc42285")
}
> db.demo76.insertOne({ Name:"Chris","Age" : 21, "CountryName" : 'AUS' });
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e2bd3e571bf0181ecc42286")
}
> db.demo76.insertOne({ Name:"Chris","Age" : 23, "CountryName" : 'AUS' });
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e2bd3e571bf0181ecc42287")
}
> db.demo76.insertOne({ Name:"Chris","Age" : 21, "CountryName" : 'UK' });
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e2bd3e571bf0181ecc42288")
}

find() 메서드를 사용하여 컬렉션의 모든 문서 표시 -

> db.demo76.find();

이것은 다음과 같은 출력을 생성합니다 -

{ "_id" : ObjectId("5e2bd3e571bf0181ecc42281"), "Name" : "Chris", "Age" : 21, "CountryName" : "US" }
{ "_id" : ObjectId("5e2bd3e571bf0181ecc42282"), "Name" : "Chris", "Age" : 21, "CountryName" : "US" }
{ "_id" : ObjectId("5e2bd3e571bf0181ecc42283"), "Name" : "Chris", "Age" : 23, "CountryName" : "UK" }
{ "_id" : ObjectId("5e2bd3e571bf0181ecc42284"), "Name" : "Chris", "Age" : 23, "CountryName" : "UK" }
{ "_id" : ObjectId("5e2bd3e571bf0181ecc42285"), "Name" : "Chris", "Age" : 21, "CountryName" : "AUS" }
{ "_id" : ObjectId("5e2bd3e571bf0181ecc42286"), "Name" : "Chris", "Age" : 21, "CountryName" : "AUS" }
{ "_id" : ObjectId("5e2bd3e571bf0181ecc42287"), "Name" : "Chris", "Age" : 23, "CountryName" : "AUS" }
{ "_id" : ObjectId("5e2bd3e571bf0181ecc42288"), "Name" : "Chris", "Age" : 21, "CountryName" : "UK" }

다음은 다단계 $group −

> db.demo76.aggregate([
... { "$match": { "Name": "Chris" }},
... { "$group": {
...    "_id": { "Age": "$Age", "CountryName": "$CountryName" },
...    "count": { "$sum": 1 }
... }},
... { "$group": {
... "_id": "$_id.Age",
... "CountryName": {
...    "$push": {
...       "k": "$_id.CountryName",
...       "v": "$count"
...       }
...    }
... }},
... { "$addFields": {
...    "CountryName": {
...       "$arrayToObject": "$CountryName"
...       }
...    }}
... ]);

이것은 다음과 같은 출력을 생성합니다 -

{ "_id" : 23, "CountryName" : { "AUS" : 1, "UK" : 2 } }
{ "_id" : 21, "CountryName" : { "UK" : 1, "AUS" : 2, "US" : 2 } }