MongoDB SELECT COUNT GROUP BY

Question

This would be the easier way to do it using aggregate:

db.contest.aggregate([
    {"$group" : {_id:"$province", count:{$sum:1}}}
])

I need some extra operation based on the result of aggregate function. Finally I've found some solution for aggregate function and the operation based on the result in MongoDB. I've a collection Request with field request, source, status, requestDate.

Single Field Group By & Count:

db.Request.aggregate([
    {"$group" : {_id:"$source", count:{$sum:1}}}
])

Multiple Fields Group By & Count:

db.Request.aggregate([
    {"$group" : {_id:{source:"$source",status:"$status"}, count:{$sum:1}}}
])

Multiple Fields Group By & Count with Sort using Field:

db.Request.aggregate([
    {"$group" : {_id:{source:"$source",status:"$status"}, count:{$sum:1}}},
    {$sort:{"_id.source":1}}
])

Multiple Fields Group By & Count with Sort using Count:

db.Request.aggregate([
    {"$group" : {_id:{source:"$source",status:"$status"}, count:{$sum:1}}},
    {$sort:{"count":-1}}
])

If you need multiple columns to group by, follow this model. Here I am conducting a count by status and type:

  db.BusinessProcess.aggregate({
    "$group": {
        _id: {
            status: "$status",
            type: "$type"
        },
        count: {
            $sum: 1
        }
    }
   })

Starting in MongoDB 3.4, you can use the $sortByCount aggregation.

Groups incoming documents based on the value of a specified expression, then computes the count of documents in each distinct group.

https://docs.mongodb.com/manual/reference/operator/aggregation/sortByCount/

For example:

db.contest.aggregate([
    { $sortByCount: "$province" }
]);

Additionally if you need to restrict the grouping you can use:

db.events.aggregate( 
    {$match: {province: "ON"}},
    {$group: {_id: "$date", number: {$sum: 1}}}  
)