When a possible group result is necessary then aggregation is used. I would like to explain aggregation by using an example as you asked.
Let us consider, a collection "employeeDB" holds employee's records/documents. Each record/document has fields likes name, designation, salary, gender, location etc.
Sample:
name: "abc", designation: "engineer", salary:30000, gender:"female", location: "Gurgaon"
name: "xyz", designation: "engineer", salary:40000, gender:"female", location: "Gurgoan"
name: "def", designation: "engineer", salary:40000, gender:"female", location: "Hyderabad"
name: "ghi", designation: "engineer", salary:45000, gender:"male", location: "Bangalore"
name: "jkl", designation: "engineer", salary:40000, gender:"male", location: "Chennai"
name: "mno", designation: "engineer", salary:40000, gender:"male", location: "Chennai"
... and so on.
Now if you want to know average salary of engineer per location then you have to use aggregate function on location field.
It will be like that.
db.employeeDB.aggregate({ $group : { _id: "$location", averageSalary : {$avg : "$salary"} } })
