MongoDB查询（聚合框架）以匹配特定的字段值
> db.demo555.insertOne({"CountryName":"US"});{
"acknowledged" : true, "insertedId" : ObjectId("5e8f21bf54b4472ed3e8e85f")
}
> db.demo555.insertOne({"CountryName":"UK"});{
"acknowledged" : true, "insertedId" : ObjectId("5e8f21c254b4472ed3e8e860")
}
> db.demo555.insertOne({"CountryName":"US"});{
"acknowledged" : true, "insertedId" : ObjectId("5e8f21c354b4472ed3e8e861")
}
> db.demo555.insertOne({"CountryName":"AUS"});{
"acknowledged" : true, "insertedId" : ObjectId("5e8f21c554b4472ed3e8e862")
}
> db.demo555.insertOne({"CountryName":"US"});{
"acknowledged" : true, "insertedId" : ObjectId("5e8f21c754b4472ed3e8e863")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo555.find();
这将产生以下输出：-
{ "_id" : ObjectId("5e8f21bf54b4472ed3e8e85f"), "CountryName" : "US" }
{ "_id" : ObjectId("5e8f21c254b4472ed3e8e860"), "CountryName" : "UK" }
{ "_id" : ObjectId("5e8f21c354b4472ed3e8e861"), "CountryName" : "US" }
{ "_id" : ObjectId("5e8f21c554b4472ed3e8e862"), "CountryName" : "AUS" }
{ "_id" : ObjectId("5e8f21c754b4472ed3e8e863"), "CountryName" : "US" }以下是匹配特定字段值的查询-
> db.demo555.aggregate([
... {$match: {CountryName: 'US'}},
... {$group: {_id: null, Total: {$sum: 1}}}
... ])这将产生以下输出-
{ "_id" : null, "Total" : 3 }