MongoDB Query to select records having a given key?

To select records having a given key, you can use the $exists operator. This operator checks whether a field exists in the document, regardless of its value.

Syntax

db.collectionName.find({ fieldName: { $exists: true } });

Sample Data

Let's create a collection with sample documents to demonstrate the $exists operator ?

db.selectRecordsHavingKeyDemo.insertMany([
    {"StudentName": "John", "StudentAge": 21, "StudentMathMarks": 78},
    {"StudentName": "Carol", "StudentMathMarks": 89},
    {"StudentName": "Sam", "StudentAge": 26, "StudentMathMarks": 89},
    {"StudentName": "Sam", "StudentMathMarks": 98}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5c8b7be780f10143d8431e0f"),
        ObjectId("5c8b7bfc80f10143d8431e10"),
        ObjectId("5c8b7c1280f10143d8431e11"),
        ObjectId("5c8b7c2180f10143d8431e12")
    ]
}

View All Documents

db.selectRecordsHavingKeyDemo.find().pretty();

{
    "_id": ObjectId("5c8b7be780f10143d8431e0f"),
    "StudentName": "John",
    "StudentAge": 21,
    "StudentMathMarks": 78
}
{
    "_id": ObjectId("5c8b7bfc80f10143d8431e10"),
    "StudentName": "Carol",
    "StudentMathMarks": 89
}
{
    "_id": ObjectId("5c8b7c1280f10143d8431e11"),
    "StudentName": "Sam",
    "StudentAge": 26,
    "StudentMathMarks": 89
}
{
    "_id": ObjectId("5c8b7c2180f10143d8431e12"),
    "StudentName": "Sam",
    "StudentMathMarks": 98
}

Example: Select Records with StudentAge Field

To find all documents that have the StudentAge field ?

db.selectRecordsHavingKeyDemo.find({ StudentAge: { $exists: true } }).pretty();

{
    "_id": ObjectId("5c8b7be780f10143d8431e0f"),
    "StudentName": "John",
    "StudentAge": 21,
    "StudentMathMarks": 78
}
{
    "_id": ObjectId("5c8b7c1280f10143d8431e11"),
    "StudentName": "Sam",
    "StudentAge": 26,
    "StudentMathMarks": 89
}

Example: Select Records without a Specific Field

To find documents that do NOT have the StudentAge field, use $exists: false ?

db.selectRecordsHavingKeyDemo.find({ StudentAge: { $exists: false } }).pretty();

{
    "_id": ObjectId("5c8b7bfc80f10143d8431e10"),
    "StudentName": "Carol",
    "StudentMathMarks": 89
}
{
    "_id": ObjectId("5c8b7c2180f10143d8431e12"),
    "StudentName": "Sam",
    "StudentMathMarks": 98
}

Conclusion

The $exists operator is useful for filtering documents based on field presence. Use $exists: true to find documents with a field, and $exists: false to find documents without it.