计算MongoDB文档中按平均值分组的平均值？
您可以将$group运算符与_id：null一起使用。以下是语法-
db.yourCollectionName.aggregate([{$group: {_id:null, "anyFieldName": {$avg:"$yourFieldName"} } }]);首先让我们创建一个包含文档的集合-
> db.caculateTheAverageValueDemo.insertOne({"Population":100});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd68a197924bb85b3f4895f")
}
> db.caculateTheAverageValueDemo.insertOne({"Population":500});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd68a1c7924bb85b3f48960")
}
> db.caculateTheAverageValueDemo.insertOne({"Population":200});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd68a237924bb85b3f48961")
}
> db.caculateTheAverageValueDemo.insertOne({"Population":100});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd68a297924bb85b3f48962")
}
> db.caculateTheAverageValueDemo.insertOne({"Population":100});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd68a2e7924bb85b3f48963")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.caculateTheAverageValueDemo.find().pretty();
这将产生以下输出-
{ "_id" : ObjectId("5cd68a197924bb85b3f4895f"), "Population" : 100 }
{ "_id" : ObjectId("5cd68a1c7924bb85b3f48960"), "Population" : 500 }
{ "_id" : ObjectId("5cd68a237924bb85b3f48961"), "Population" : 200 }
{ "_id" : ObjectId("5cd68a297924bb85b3f48962"), "Population" : 100 }
{ "_id" : ObjectId("5cd68a2e7924bb85b3f48963"), "Population" : 100 }以下是对MongoDB文档的值求平均值的查询-
> db.caculateTheAverageValueDemo.aggregate([{$group: {_id:null, "AveragePopulation": {$avg:"$Population"} } }]);这将产生以下输出-
{ "_id" : null, "AveragePopulation" : 200 }