Check for null in MongoDB?

In MongoDB, you can check for null values using the $type operator with type number 10 or alias "null". This operator specifically matches fields that contain explicit null values, not missing fields or empty strings.

Syntax

db.collection.find({"fieldName": { $type: 10 }});
// OR
db.collection.find({"fieldName": { $type: "null" }});

Sample Data

Let's create a collection with different types of values to demonstrate null checking ?

db.mongoDbEqualDemo.insertMany([
    {"Age": 34},
    {"Age": ""},
    {"Age": null},
    {"Age": 56},
    {},
    {"Age": null}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5cd7e9121a844af18acdffa3"),
        ObjectId("5cd7e9161a844af18acdffa4"),
        ObjectId("5cd7e9191a844af18acdffa5"),
        ObjectId("5cd7e91e1a844af18acdffa6"),
        ObjectId("5cd7e9261a844af18acdffa7"),
        ObjectId("5cd7e92e1a844af18acdffa8")
    ]
}

Example: Find Documents with Null Values

Display all documents to see the different data types ?

db.mongoDbEqualDemo.find().pretty();

{ "_id": ObjectId("5cd7e9121a844af18acdffa3"), "Age": 34 }
{ "_id": ObjectId("5cd7e9161a844af18acdffa4"), "Age": "" }
{ "_id": ObjectId("5cd7e9191a844af18acdffa5"), "Age": null }
{ "_id": ObjectId("5cd7e91e1a844af18acdffa6"), "Age": 56 }
{ "_id": ObjectId("5cd7e9261a844af18acdffa7") }
{ "_id": ObjectId("5cd7e92e1a844af18acdffa8"), "Age": null }

Now check for documents where Age field has explicit null values ?

db.mongoDbEqualDemo.find({"Age": { $type: 10 }});

{ "_id": ObjectId("5cd7e9191a844af18acdffa5"), "Age": null }
{ "_id": ObjectId("5cd7e92e1a844af18acdffa8"), "Age": null }

Key Points

• $type: 10 matches only explicit null values, not missing fields or empty strings.
• You can use $type: "null" instead of the numeric type 10 for better readability.
• Missing fields and empty strings ("") are not considered null by this operator.

Conclusion

Use $type: 10 or $type: "null" to find documents with explicit null values. This approach distinguishes between null values, missing fields, and empty strings in your MongoDB queries.