mongodbAggregation

https://learn.mongodb.com/learn/course/mongodb-aggregation/lesson-1-introduction-to-mongodb-aggregation/learn?page=2 

Aggregation operations process multiple documents and return computed results. You can use aggregation operations to:

• Group values from multiple documents together.

• Perform operations on the grouped data to return a single result.

• Analyze data changes over time.

To perform aggregation operations, you can use:

• Aggregation pipelines, which are the preferred method for performing aggregations.

The command to use Aggregation is:

db.collecationname.aggregate(pipeline,options)

Pipeline:

A sequence of data aggregation opeartions or stages

Pipeline is an array

filtered grouped sorted transformed

$match:

filters data that matches criteria

$Group:

Groups documents based on criteria

$sort:

puts the document in specified order 1:ascending -1 :descending.. 

$out: writes the document that are returned by an aggregation pipeline into a collection

it crates new collection if not otherwise overwrites 

Options:

Documents can be passed  as well

valid  Aggregate stages:

$count

$group

$limit 

$lookup

$match

$merge

$sort

$project

$unwind

$unset

.............

1:

var pipeline=[

    {$sort:{"rollno":1}},

    

];

db.record.aggregate(pipeline);

2:

var pipeline=[

    {$sort:{"rollno":1}},

    {$limit:2}

];

db.record.aggregate(pipeline);

3:

var pipeline=[

{$set:{

  username:{$concat:["$name","$address"]}

}},

    {$sort:{"rollno":-1}}

    

];

db.record.aggregate(pipeline);

.........................................................................................

db.records.insertMany([

    {name:'rahul',city:'delhi',zone:'north'},

    {name:'raj',city:'delhi',zone:'south'},

    {name:'rakesh',city:'delhi',zone:'north'},

    {name:'akshay',city:'delhi',zone:'north'},

    {name:'suman',city:'up',zone:'north'}

])

.................................................................

1:matches the city delhi

var pipeline=[

{$match:{

"city":"delhi"  

  }

}

    

];

db.records.aggregate(pipeline);

2:

var pipeline=[

{$match:{

"city":"delhi"  

  }},

{$group:{

    _id:"$zone",

    name:{$count:{}}

}}

    

];

db.records.aggregate(pipeline);

..............................................

3.

var pipeline=[

{$project:{

name:1,

zone:1,

"state":"$city"  

  }}

    

];

db.records.aggregate(pipeline);

4.

var pipeline=[

{$count:"name" 

  }

    

];

db.records.aggregate(pipeline);

5...

var pipeline=[

{$count:"name" 

  },

{$out:"totalname"}

    

];

db.records.aggregate(pipeline);