mongodb count num of distinct values per field/key
Here is example of using aggregation API. To complicate the case we're grouping by case-insensitive words from array property of the document.
db.articles.aggregate([
{
$match: {
keywords: { $not: {$size: 0} }
}
},
{ $unwind: "$keywords" },
{
$group: {
_id: {$toLower: '$keywords'},
count: { $sum: 1 }
}
},
{
$match: {
count: { $gte: 2 }
}
},
{ $sort : { count : -1} },
{ $limit : 100 }
]);
that give result such as
{ "_id" : "inflammation", "count" : 765 }
{ "_id" : "obesity", "count" : 641 }
{ "_id" : "epidemiology", "count" : 617 }
{ "_id" : "cancer", "count" : 604 }
{ "_id" : "breast cancer", "count" : 596 }
{ "_id" : "apoptosis", "count" : 570 }
{ "_id" : "children", "count" : 487 }
{ "_id" : "depression", "count" : 474 }
{ "_id" : "hiv", "count" : 468 }
{ "_id" : "prognosis", "count" : 428 }
MongoDB has a distinct
command which returns an array of distinct values for a field; you can check the length of the array for a count.
There is a shell db.collection.distinct()
helper as well:
> db.countries.distinct('country');
[ "Spain", "England", "France", "Australia" ]
> db.countries.distinct('country').length
4
As noted in the MongoDB documentation:
Results must not be larger than the maximum BSON size (16MB). If your results exceed the maximum BSON size, use the aggregation pipeline to retrieve distinct values using the
$group
operator, as described in Retrieve Distinct Values with the Aggregation Pipeline.
With MongoDb 3.4.4 and newer, you can leverage the use of $arrayToObject
operator and a $replaceRoot
pipeline to get the counts.
For example, suppose you have a collection of users with different roles and you would like to calculate the distinct counts of the roles. You would need to run the following aggregate pipeline:
db.users.aggregate([
{ "$group": {
"_id": { "$toLower": "$role" },
"count": { "$sum": 1 }
} },
{ "$group": {
"_id": null,
"counts": {
"$push": { "k": "$_id", "v": "$count" }
}
} },
{ "$replaceRoot": {
"newRoot": { "$arrayToObject": "$counts" }
} }
])
Example Output
{
"user" : 67,
"superuser" : 5,
"admin" : 4,
"moderator" : 12
}
You can leverage on Mongo Shell Extensions. It's a single .js import that you can append to your $HOME/.mongorc.js
, or programmatically, if you're coding in Node.js/io.js too.
Sample
For each distinct value of field counts the occurrences in documents optionally filtered by query
>
db.users.distinctAndCount('name', {name: /^a/i})
{
"Abagail": 1,
"Abbey": 3,
"Abbie": 1,
...
}
The field parameter could be an array of fields
>
db.users.distinctAndCount(['name','job'], {name: /^a/i})
{
"Austin,Educator" : 1,
"Aurelia,Educator" : 1,
"Augustine,Carpenter" : 1,
...
}