Data Structure
Networking
RDBMS
Operating System
Java
MS Excel
iOS
HTML
CSS
Android
Python
C Programming
C++
C#
MongoDB
MySQL
Javascript
PHP
Physics
Chemistry
Biology
Mathematics
English
Economics
Psychology
Social Studies
Fashion Studies
Legal Studies
- Selected Reading
- UPSC IAS Exams Notes
- Developer's Best Practices
- Questions and Answers
- Effective Resume Writing
- HR Interview Questions
- Computer Glossary
- Who is Who
MongoDB $unwind to get the count
The $unwind in MongoDB deconstructs an array field from the input documents to output a document for each element. Use $unwind along with aggregate() to get the count. Let us create a collection with documents −
> 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")
}Display all documents from a collection with the help of find() method −
> db.demo478.find();
This will produce the following output −
{ "_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 } ] } }Following is the query to implement $unwind to get the count −
> db.demo478.aggregate([
... { "$unwind": "$Details.Information" },
... {
... "$group" : {
... "_id": "$Details.Information.Age",
... "count" : {
... "$sum": {
... "$cond": [
... { "$gt": [ "$Details.Information.Name", null ] },
... 1, 0
... ]
... }
... }
... }
... },
... { "$sort" : { "count" : -1 } }
... ])This will produce the following output −
{ "_id" : 21, "count" : 2 }
{ "_id" : 23, "count" : 1 }
{ "_id" : 24, "count" : 0 }
{ "_id" : 20, "count" : 0 }
{ "_id" : 22, "count" : 0 }
Updated on: 11-May-2020
453 Views
Advertisements