You can use the following syntax to query for all documents where a specific field is not null in MongoDB:

db.collection.find({"field_name":{$ne:null}}) 

The following examples show how to use this syntax in practice.

Example 1: Query for “not null” in Specific Field

Suppose we have a collection teams with the following documents:

db.teams.insertOne({team: "Mavs", position: null, points: 31})
db.teams.insertOne({team: "Spurs", position: "Guard", points: 22})
db.teams.insertOne({team: "Rockets", position: null, points: 19})
db.teams.insertOne({team: "Warriors", position: "Forward", points: 26})
db.teams.insertOne({team: "Cavs", position: "Guard", points: 33})

We can use the following code to find all documents where the “position” field is not null:

db.teams.find({"position":{$ne:null}})

This query returns the following documents:

{ _id: ObjectId("618bf18f35d8a762d3c28717"),
  team: 'Spurs',
  position: 'Guard',
  points: 22 }

{ _id: ObjectId("618bf18f35d8a762d3c28719"),
  team: 'Warriors',
  position: 'Forward',
  points: 26 }

{ _id: ObjectId("618bf18f35d8a762d3c2871a"),
  team: 'Cavs',
  position: 'Guard',
  points: 33 }

Notice that the only documents returned are the ones where the “position” field is not null.

Example 2: Query for “not null” (When Not Every Document Contains the Field)

Suppose we have a collection teams with the following documents:

db.teams.insertOne({team: "Mavs", position: null, points: 31})
db.teams.insertOne({team: "Spurs", points: 22})
db.teams.insertOne({team: "Rockets", position: null, points: 19})
db.teams.insertOne({team: "Warriors", position: "Forward", points: 26})
db.teams.insertOne({team: "Cavs", position: "Guard", points: 33})

Note that the second document in the collection doesn’t even have a “position” field.

We can use the following code to find all documents where the “position” field is not null:

db.teams.find({"position":{$ne:null}})

This query returns the following documents:

{ _id: ObjectId("618bf18f35d8a762d3c28719"),
  team: 'Warriors',
  position: 'Forward',
  points: 26 }

{ _id: ObjectId("618bf18f35d8a762d3c2871a"),
  team: 'Cavs',
  position: 'Guard',
  points: 33 }

Since the second document doesn’t even have a “position” field, it is not returned.

Also note that the other two documents that have a null value in the “position field are not returned either.

Summary: By using the $ne:null syntax, we only return the documents where a specific field exists and is not null.

Additional Resources

The following tutorials explain how to perform other common operations in MongoDB:

MongoDB: How to Query with “Like” Regex
MongoDB: How to Check if Field Contains a String
MongoDB: How to Add a New Field in a Collection
MongoDB: How to Remove a Field from Every Document