In a regular SQL database, you can check if a column is null by using IS NULL
For example if you wanted to return all rows where the age is null, you would do the following

FROM SomeTable
FROM SomeTable

In a NoSQL database it is possible that half the documents in a collection are omitted and maybe five are there with the value null. How can you know if the field is missing or has the value null?

Let’s take a quick look. First insert the following document into your collection

db.Blog.insert( { name : "Denis2" } )

As you can see it just has a name. Now let’s add another document this time with age as well, we will make the age NULL

db.Blog.insert( { name : "Denis",  age : NULL} )

You get the following error
Sun Feb 17 13:44:58 ReferenceError: NULL is not defined (shell):1

This error occurs because you need to pass null in lowercase

db.Blog.insert( { name : "Denis",  age : null } )

Now if you execute the following


You get back both document
{ “_id” : ObjectId(“512118a7c1eca3d7ffcd00f9”), “name” : “Denis”, “age” : null }
{ “_id” : ObjectId(“512123d9c1eca3d7ffcd00fa”), “name” : “Denis2” }

In order to return the document where the value that is stored is null, you can use $type: 10. What that means is that the field is of BSON Type Null

db.Blog.find( { age: { $type: 10 } } )

Here is the output

{ "_id" : ObjectId("512118a7c1eca3d7ffcd00f9"), "name" : "Denis", "age" : null }

In order to return the document where the field does not exist, you can use $exists: false

db.Blog.find( { age: { $exists: false } } )

Here is the output

{ "_id" : ObjectId("512123d9c1eca3d7ffcd00fa"), "name" : "Denis2" }

That is all for this post, if you are interested in my other MongoDB posts, you can find them here:
Install MongoDB as a Windows Service
UPSERTs with MongoDB
How to sort results in MongoDB
Indexes in MongoDB: A quick overview
Multidocument updates with MongoDB
MongoDB: How to include and exclude the fields you want in results
MongoDB: How to limit results and how to page through results
MongoDB: How to backup and restore databases
MongoDB: How to restore collections
MongoDB: How to backup all the databases with one command
MongoDB: Exporting data into files
MongoDB: How to drop databases and collections
MongoDB: Creating capped collections