How to check empty field in a MongoDB collection?

To check empty field in a MongoDB collection, use $exists along with $eq operator. Let us create a collection with documents −

> db.demo485.insertOne({"FirstName":"Chris","LastName":""});{    "acknowledged" : true,    "insertedId" : ObjectId("5e82e9f6b0f3fa88e22790a8") } > db.demo485.insertOne({"FirstName":"David","LastName":"Miller"});{    "acknowledged" : true,    "insertedId" : ObjectId("5e82e9fdb0f3fa88e22790a9") } > db.demo485.insertOne({"FirstName":"Chris","LastName":"Brown"});{    "acknowledged" : true,    "insertedId" : ObjectId("5e82ea03b0f3fa88e22790aa") } > db.demo485.insertOne({"FirstName":"Robert","LastName":""});{    "acknowledged" : true,    "insertedId" : ObjectId("5e82ea0fb0f3fa88e22790ab") }

Display all documents from a collection with the help of find() method −

> db.demo485.find();

This will produce the following output −

{ "_id" : ObjectId("5e82e9f6b0f3fa88e22790a8"), "FirstName" : "Chris", "LastName" : "" } { "_id" : ObjectId("5e82e9fdb0f3fa88e22790a9"), "FirstName" : "David", "LastName" : "Miller" } { "_id" : ObjectId("5e82ea03b0f3fa88e22790aa"), "FirstName" : "Chris", "LastName" : "Brown" } { "_id" : ObjectId("5e82ea0fb0f3fa88e22790ab"), "FirstName" : "Robert", "LastName" : "" }

Following is the query to check empty field −

> db.demo485.find({"LastName" : {"$exists" : true, "$eq" : ""}})

This will produce the following output −

{ "_id" : ObjectId("5e82e9f6b0f3fa88e22790a8"), "FirstName" : "Chris", "LastName" : "" } { "_id" : ObjectId("5e82ea0fb0f3fa88e22790ab"), "FirstName" : "Robert", "LastName" : "" }