How do I check whether a field contains null value in MongoDB?

You can use $type operator to check whether a filed contains null value. Let us first create a collection with documents. We have also inserted a null in a field −

> db.nullDemo.insertOne({"FirstName":"Chris"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cc68a1eac184d684e3fa270")
}
> db.nullDemo.insertOne({"FirstName":null});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cc68a25ac184d684e3fa271")
}
> db.nullDemo.insertOne({"FirstName":"Robert"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cc68a2cac184d684e3fa272")
}

Following is the query to display all documents from a collection with the help of find() method −

> db.nullDemo.find().pretty();

This will produce the following output. One of the field is null −

{ "_id" : ObjectId("5cc68a1eac184d684e3fa270"), "FirstName" : "Chris" }
{ "_id" : ObjectId("5cc68a25ac184d684e3fa271"), "FirstName" : null }
{ "_id" : ObjectId("5cc68a2cac184d684e3fa272"), "FirstName" : "Robert" }

Here is the query to check whether a field contains null value. The field “FirstName” is checked −

> db.nullDemo.find( { FirstName: { $type: 10 } } );

This will produce the following output −

{ "_id" : ObjectId("5cc68a25ac184d684e3fa271"), "FirstName" : null }