选择MongoDB中按字段分组的文档？
要选择按MongoDB中的字段分组的文档，请使用$group和$project。首先让我们创建一个包含文档的集合-
> db.demo540.insertOne({id:1,"Name":"Chris","CountryName":"US"});{
"acknowledged" : true, "insertedId" : ObjectId("5e8ca368ef4dcbee04fbbc0e")
}
> db.demo540.insertOne({id:1,"Name":"Chris","CountryName":"UK"});{
"acknowledged" : true, "insertedId" : ObjectId("5e8ca36bef4dcbee04fbbc0f")
}
> db.demo540.insertOne({id:1,"Name":"Chris","CountryName":"AUS"});{
"acknowledged" : true, "insertedId" : ObjectId("5e8ca370ef4dcbee04fbbc10")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo540.find();
这将产生以下输出-
{ "_id" : ObjectId("5e8ca368ef4dcbee04fbbc0e"), "id" : 1, "Name" : "Chris", "CountryName" :
"US" }
{ "_id" : ObjectId("5e8ca36bef4dcbee04fbbc0f"), "id" : 1, "Name" : "Chris", "CountryName" :
"UK" }
{ "_id" : ObjectId("5e8ca370ef4dcbee04fbbc10"), "id" : 1, "Name" : "Chris", "CountryName" :
"AUS" }以下是查询以选择按字段分组的文档-
> var query= [
... {
... "$group": {
... "_id": {
... "id": "$id",
... "Name": "$Name"
... },
... "details": {
... "$push": {
...
... "CountryName": "$CountryName"
...
... }
... }
... }
... },
... {
... "$project": {
... "_id": "$_id.id",
... "Name": "$_id.Name",
... "details": 1
... }
... }
... ];
> db.demo540.aggregate(q).pretty();这将产生以下输出-
{
"details" : [
{
"Name" : "Chris",
"CountryName" : "US"
},
{
"Name" : "Chris",
"CountryName" : "UK"
},
{
"Name" : "Chris",
"CountryName" : "AUS"
}
],
"_id" : 1,
"Name" : "Chris"
}