MongoDB $ unwind获得计数
MongoDB中的$unwind从输入文档中解构一个数组字段，以输出每个元素的文档。使用$unwind和aggregate()来获取计数。让我们创建一个包含文档的集合-
> db.demo478.insertOne(
... {
...
... "Details" : {
... _id:1,
... "Information" : [
... {
... "Name" : "Chris",
... "Age":21
... },
... {
... "Name" : "David",
... "Age":23
... },
... {
...
... "Name" : null,
... "Age":22
... },
... {
...
... "Name" : null,
... "Age":24
... }
... ]
... }
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5e8204acb0f3fa88e2279092")
}
>
> db.demo478.insertOne(
... {
...
... "Details" : {
... _id:2,
... "Information" : [
... {
... "Name" : "Bob",
... "Age":21
... },
... {
... "Name" : null,
... "Age":20
... }
... ]
... }
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5e8204adb0f3fa88e2279093")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo478.find();
这将产生以下输出-
{ "_id" : ObjectId("5e8204acb0f3fa88e2279092"), "Details" : { "_id" : 1, "Information" : [ {
"Name" : "Chris", "Age" : 21 }, { "Name" : "David", "Age" : 23 }, { "Name" : null, "Age" : 22 }, {
"Name" : null, "Age" : 24 } ] } }
{ "_id" : ObjectId("5e8204adb0f3fa88e2279093"), "Details" : { "_id" : 2, "Information" : [ {
"Name" : "Bob", "Age" : 21 }, { "Name" : null, "Age" : 20 } ] } }以下是实现$unwind以获取计数的查询-
> db.demo478.aggregate([
... { "$unwind": "$Details.Information" },
... {
... "$group" : {
... "_id": "$Details.Information.Age",
... "count" : {
... "$sum": {
... "$cond": [
... { "$gt": [ "$Details.Information.Name", null ] },
... 1, 0
... ]
... }
... }
... }
... },
... { "$sort" : { "count" : -1 } }
... ])这将产生以下输出-
{ "_id" : 21, "count" : 2 }
{ "_id" : 23, "count" : 1 }
{ "_id" : 24, "count" : 0 }
{ "_id" : 20, "count" : 0 }
{ "_id" : 22, "count" : 0 }