MongoDB equivalent of `select distinct(name) from collectionName where age = "25"`?

To get the MongoDB equivalent of select distinct(name) from collectionName where age = "25", use the distinct() method with a field name and query condition. This returns unique values from the specified field that match the given criteria.

Syntax

db.collection.distinct("fieldName", { "conditionField": value })

Sample Data

db.distinctNameAndAgeDemo.insertMany([
    { "ClientFirstName": "John", "Age": 23 },
    { "ClientFirstName": "Larry", "Age": 25 },
    { "ClientFirstName": "David", "Age": 25 },
    { "ClientFirstName": "Carol", "Age": 26 },
    { "ClientFirstName": "Sam", "Age": 25 },
    { "ClientFirstName": "Larry", "Age": 25 },
    { "ClientFirstName": "Carol", "Age": 26 }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5cd12759e3526dbddbbfb60b"),
        ObjectId("5cd12768e3526dbddbbfb60c"),
        ObjectId("5cd12773e3526dbddbbfb60d"),
        ObjectId("5cd1277ee3526dbddbbfb60e"),
        ObjectId("5cd12793e3526dbddbbfb60f"),
        ObjectId("5cd127a3e3526dbddbbfb610"),
        ObjectId("5cd127aae3526dbddbbfb611")
    ]
}

Display All Documents

db.distinctNameAndAgeDemo.find().pretty();

{
    "_id": ObjectId("5cd12759e3526dbddbbfb60b"),
    "ClientFirstName": "John",
    "Age": 23
}
{
    "_id": ObjectId("5cd12768e3526dbddbbfb60c"),
    "ClientFirstName": "Larry",
    "Age": 25
}
{
    "_id": ObjectId("5cd12773e3526dbddbbfb60d"),
    "ClientFirstName": "David",
    "Age": 25
}
{
    "_id": ObjectId("5cd1277ee3526dbddbbfb60e"),
    "ClientFirstName": "Carol",
    "Age": 26
}
{
    "_id": ObjectId("5cd12793e3526dbddbbfb60f"),
    "ClientFirstName": "Sam",
    "Age": 25
}
{
    "_id": ObjectId("5cd127a3e3526dbddbbfb610"),
    "ClientFirstName": "Larry",
    "Age": 25
}
{
    "_id": ObjectId("5cd127aae3526dbddbbfb611"),
    "ClientFirstName": "Carol",
    "Age": 26
}

Get Distinct Names Where Age = 25

The following query is the MongoDB equivalent of select distinct(name) from collectionName where age = "25" ?

db.distinctNameAndAgeDemo.distinct("ClientFirstName", { "Age": 25 });

[ "Larry", "David", "Sam" ]

Key Points

• The distinct() method automatically removes duplicates (notice "Larry" appears only once despite having two documents)
• The second parameter is the query filter - only documents matching this condition are considered
• Results are returned as an array of unique values

Conclusion

MongoDB's distinct() method provides SQL-like distinct functionality. It returns unique field values that match the specified query condition, automatically filtering out duplicates.