Is there a query for calculating how many distinct values a field contains in DB.
f.e I have a field for country and there are 8 types of country values (spain, england, france, etc...)
If someone adds more documents with a new country I would like the query to return 9.
Is there easier way then group and count?
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
$groupoperator, as described in Retrieve Distinct Values with the Aggregation Pipeline.
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 }
unwind is necessary because the code is grouping individual values of an array field which matches how distinct works.
unwind is necessary when working with arrays.
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
}
.distinct().I wanted a more concise answer and I came up with the following using the documentation at aggregates and group
db.countries.aggregate([{"$group": {"_id": "$country", "count":{"$sum": 1}}}])
You can leverage on Mongo Shell Extensions. It's a single .js import that you can append to your $HOME/.mongorc.js, or programmatically, if you're coding in Node.js/io.js too.
Sample
For each distinct value of field counts the occurrences in documents optionally filtered by query
>
db.users.distinctAndCount('name', {name: /^a/i})
{
"Abagail": 1,
"Abbey": 3,
"Abbie": 1,
...
}
The field parameter could be an array of fields
>
db.users.distinctAndCount(['name','job'], {name: /^a/i})
{
"Austin,Educator" : 1,
"Aurelia,Educator" : 1,
"Augustine,Carpenter" : 1,
...
}
require("./script.js"), i suppose
.mongorc.jsfile into your home dir. Done.To find distinct in field_1 in collection but we want some WHERE condition too than we can do like following :
db.your_collection_name.distinct('field_1', {WHERE condition here and it should return a document})
So, find number distinct names from a collection where age > 25 will be like :
db.your_collection_name.distinct('names', {'age': {"$gt": 25}})
Hope it helps!
I use this query:
var collection = "countries"; var field = "country";
db[collection].distinct(field).forEach(function(value){print(field + ", " + value + ": " + db[collection].count({[field]: value}))})
Output:
countries, England: 3536
countries, France: 238
countries, Australia: 1044
countries, Spain: 16
This query first distinct all the values, and then count for each one of them the number of occurrences.
hosts was the name of my collection in my db... sorry for that. The other issue that I also fixed tried to call db.collection which I fixed to db[collection]. Thanks!
If you're on MongoDB 3.4+, you can use $count in an aggregation pipeline:
db.users.aggregate([
{ $group: { _id: '$country' } },
{ $count: 'countOfUniqueCountries' }
]);
Case 1 If you are using find() then the solution for that is:
const getDistinctCountryCount = async (req, res) => {
const countryCount = await(await db.model.distinct('country')).length;
console.log("countryCount: ", countryCount);
}
You have to first await the query result, then await to get the length of it.
Case 2 If you are using Aggregate then the solution for that is:
db.model.aggregate([
{ $group: { _id: '$country' } },
{ $count: 'countOfDistinchCountries' }
])
Shortest query I've found to get a sorted count of all distincts:
db.myCollection.aggregate([{
"$group": {"_id": "$myField", "count":{"$sum": 1}}
},
{ "$sort": {"count": -1}}
])
