MongoDB Count Distinct

MongoDB provides a convenient way to count the number of distinct documents in a collection or field using the distinct method.

Syntax

db.collection.distinct(fieldName, query)

Where:

• fieldName: the name of the field to count distinct values in.
• query (optional): the selection criteria for the documents to count. This can be used to filter the documents before counting the distinct values.

Example

Given the following documents in a users collection:

[
  { name: "Alice", age: 25, gender: "female" },
  { name: "Bob", age: 30, gender: "male" },
  { name: "Charlie", age: 25, gender: "male" },
  { name: "Dave", age: 35, gender: "male" },
  { name: "Eve", age: 30, gender: "female" }
]

To count the number of distinct ages in the collection, we can use the following command in the MongoDB shell:

db.users.distinct("age")

This will return an array of distinct ages:

[25, 30, 35]

We can also count the number of distinct genders for users over the age of 25:

db.users.distinct("gender", { age: { $gt: 25 } })

This will return an array of gender values for users over the age of 25:

["male", "female"]

Conclusion

MongoDB's distinct method provides a simple way to count the number of distinct values in a collection or field. By using the query parameter, we can also filter the documents before counting the distinct values.