在包含多个文档的MongoDB集合中按国家，州和城市进行汇总
聚合操作将来自多个文档的值分组在一起，并且可以对分组的数据执行各种操作以返回单个结果。
要在MongoDB中聚合，请使用aggregate()。让我们创建一个包含文档的集合-
> db.demo620.insertOne({"Country":"IND","City":"Delhi",state:"Delhi"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e9a8de96c954c74be91e6a1")
}
> db.demo620.insertOne({"Country":"IND","City":"Bangalore",state:"Karnataka"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e9a8e336c954c74be91e6a3")
}
> db.demo620.insertOne({"Country":"IND","City":"Mumbai",state:"Maharashtra"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e9a8e636c954c74be91e6a4")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo620.find();
这将产生以下输出-
{ "_id" : ObjectId("5e9a8de96c954c74be91e6a1"), "Country" : "IND", "City" : "Delhi", "state" : "Delhi" }
{ "_id" : ObjectId("5e9a8e336c954c74be91e6a3"), "Country" : "IND", "City" : "Bangalore", "state" : "Karnataka" }
{ "_id" : ObjectId("5e9a8e636c954c74be91e6a4"), "Country" : "IND", "City" : "Mumbai", "state" : "Maharashtra" }以下是按国家，州和城市汇总的查询-
> db.demo620.aggregate([
... { "$group": {
... "_id": {
... "Country": "$Country",
... "state": "$state"
... },
... "City": {
... "$addToSet": {
... "City": "$City"
... }
... }
... }},
... { "$group": {
... "_id": "$_id.Country",
... "states": {
... "$addToSet": {
... "state": "$_id.state",
... "City": "$City"
... }
... }
... }}
... ]).pretty();这将产生以下输出-
{
"_id" : "IND",
"states" : [
{
"state" : "Delhi",
"City" : [
{
"City" : "Delhi"
}
]
},
{
"state" : "Maharashtra",
"City" : [
{
"City" : "Mumbai"
}
]
},
{
"state" : "Karnataka",
"City" : [
{
"City" : "Bangalore"
}
]
}
]
}