MongoDB - Aggregation

Aggregation operations process records of data and return calculated results. Aggregation operations group values ​​from multiple documents together and can perform different operations on the grouped data to return the same result. In SQL, count(*) and with group by is the equivalent of mongodb aggregation.

Aggregate() method

For aggregation in MongoDB, you must use the aggregate() method .

Syntax

The basic syntax of the aggregate() method is as follows:

>db.COLLECTION_NAME.aggregate(AGGREGATE_OPERATION)

example

In the collection, you have the following data −

{ 
   _id : ObjectId ( 7df78ad8902c ) 
   title : 'MongoDB Overview' ,  
   description : 'MongoDB is no sql database' , 
   by_user : 'tutorials point' , 
   url : 'http://www.tutorialspoint.com' , 
   tags : [ 'mongodb ' , 'database' , 'NoSQL' ], 
   likes : 100 }, { 
   _id : ObjectId ( 7df78ad8902d ) 
   title :         

  'NoSQL Overview' ,  
   description : 'No sql database is very fast' , 
   by_user : 'tutorials point' , 
   url : 'http://www.tutorialspoint.com' , 
   tags : [ 'mongodb' , 'database' , 'NoSQL ' ], 
   likes : 10 }, { 
   _id : ObjectId ( 7df78ad8902e ) 
   title : 'Neo4j Overview' ,  
   description : 'Neo4j is no sql database' ,       

   
   by_user : 'Neo4j' , 
   url : 'http://www.neo4j.com' , 
   tags : [ 'neo4j' , 'database' , 'NoSQL' ], 
   likes : 750 },      

Now from the above collection, if you want to display a list showing the number of tutorials written by each user, you will use the following aggregate() method:

> db . mycol . aggregate ([{ $group : { _id : "$by_user" , num_tutorial : { $sum : 1 }}}]) { "result" : [ { "_id" : "tutorials point" , "num_tutorial" : 2 }, { "_id" : "Neo4j" , "num_tutorial" : 1 } ], "ok" : 1 } >    

The sql equivalent query for the above use case will select by_user , count (*) from the group mycol by by_user .

In the example above, we have grouped the documents by the by_user field, and each time by_user occurs, the previous value of the sum is incremented. Below is a list of available aggregation expressions.