Do not use $ symbol or period (.) because these characters are not allowed for MongoDB field names. The field shouldn’t start with $.Here is an example of the allowed characters −> db.charactersAllowedDemo.insertOne({"Employee Name" : "John"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7fefbc8d10a061296a3c6d") }Display all documents from a collection with the help of find() method. The query is as follows −> db.charactersAllowedDemo.find().pretty();The following is the output −{    "_id" : ObjectId("5c7fefbc8d10a061296a3c6d"),    "Employee Name" : "John" }

There is no difference between count() and find().count(). Let us see how both of them works. To understand the concept, let us create a collection with the document. The query to create a collection with a document is as follows −> db.countDemo.insertOne({"UserId":1, "UserName":"John"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f9d278d10a061296a3c5d") } > db.countDemo.insertOne({"UserId":2, "UserName":"Carol"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f9d308d10a061296a3c5e") } > db.countDemo.insertOne({"UserId":3, "UserName":"Bob"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f9d3a8d10a061296a3c5f") } > db.countDemo.insertOne({"UserId":4, "UserName":"Mike"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f9d428d10a061296a3c60") }Display all documents from a collection with the help ... Read More

Use $in operator to query an array of arrays in MongoDB. To understand the concept, let us create a collection with the document. The query to create a collection with a document is as follows −> db.arrayOfArraysDemo.insertOne({"EmployeeName":"Larry", "EmployeeSkills":[["Java", "MongoDB", "MySQL", "SQL Server"]]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f7a8d8d10a061296a3c5b") } > db.arrayOfArraysDemo.insertOne({"EmployeeName":"Mike", "EmployeeSkills":[["C", "C++"]]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f7aa68d10a061296a3c5c") }Display all documents from a collection with the help of find() method. The query is as follows −> db.arrayOfArraysDemo.find().pretty();The following is the output −{    "_id" : ObjectId("5c7f7a8d8d10a061296a3c5b"),    "EmployeeName" : "Larry",    "EmployeeSkills" : ... Read More

Use $nin operator along with $elemMatch and $not for this. To understand the concept, let us create a collection with the document. The query to create a collection with a document is as follows −> db.combinationOfArrayDemo.insertOne({"StudentName":"Larry", "StudentAge":21, "StudentFavouriteTechnicalSubject":["C", "Java"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f77cc8d10a061296a3c58") } > db.combinationOfArrayDemo.insertOne({"StudentName":"Mike", "StudentAge":23, "StudentFavouriteTechnicalSubject":["C++", "Java"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f77dc8d10a061296a3c59") } > db.combinationOfArrayDemo.insertOne({"StudentName":"David", "StudentAge":22, "StudentFavouriteTechnicalSubject":["Java"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f77f48d10a061296a3c5a") }Display all documents from a collection with the help of find() method. The query is as follows −> db.combinationOfArrayDemo.find().pretty();The following is the output ... Read More

You can perform distinct with multiple keys with the help of an aggregate framework.To understand the concept, let us create a collection with the document. The query to create a collection with a document is as follows −> db.distinctWithMultipleKeysDemo.insertOne({"StudentName":"Mike", "StudentAge":22, "StudentMathMarks":56}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f74488d10a061296a3c53") } > db.distinctWithMultipleKeysDemo.insertOne({"StudentName":"Mike", "StudentAge":22, "StudentMathMarks":56}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f744b8d10a061296a3c54") } > db.distinctWithMultipleKeysDemo.insertOne({"StudentName":"Bob", "StudentAge":23, "StudentMathMarks":45}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f74598d10a061296a3c55") } > db.distinctWithMultipleKeysDemo.insertOne({"StudentName":"Bob", "StudentAge":23, "StudentMathMarks":45}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f745e8d10a061296a3c56") } > db.distinctWithMultipleKeysDemo.insertOne({"StudentName":"Carol", "StudentAge":27, "StudentMathMarks":54}); {    "acknowledged" ... Read More

Use regexp to make case-insensitive queries on MongoDB. To understand the concept, let us create a collection with the document. The query to create a collection with a document is as follows −> db.caseInsensitiveDemo.insertOne({"UserName":"David"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f6fec8d10a061296a3c45") } > db.caseInsensitiveDemo.insertOne({"UserName":"DAVID"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f6ff28d10a061296a3c46") } > db.caseInsensitiveDemo.insertOne({"UserName":"david"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f6ffa8d10a061296a3c47") } > db.caseInsensitiveDemo.insertOne({"UserName":"Carol"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f70008d10a061296a3c48") } > db.caseInsensitiveDemo.insertOne({"UserName":"Mike"}); { "acknowledged" : true, "insertedId" : ObjectId("5c7f70058d10a061296a3c49") } > db.caseInsensitiveDemo.insertOne({"UserName":"Sam"}); {    "acknowledged" : true,    "insertedId" : ... Read More

To remove an element from a doubly-nested array in MongoDB document, you can use $pull operator.To understand the concept, let us create a collection with the document. The query to create a collection with a document is as follows −> db.removeElementFromDoublyNestedArrayDemo.insertOne(    ... {       ... "_id" : "1",       ... "UserName" : "Larry",       ... "UserDetails" : [          ... {             ... "UserCountryName" : "US",             ... "UserLocation" : [                ... { ... Read More

You can achieve this with the help of $addFields operator. To understand the concept, let us create a collection with the document. The query to create a collection with a document is as follows −> db.addFieldDemo.insertOne({"EmployeeId":101, "EmployeeName":"Larry", "EmployeeDetails":{    "EmployeeSalary":65000, "EmployeeCity":"New York", "Message":"Hi"}}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f654d8d10a061296a3c44") }Display all documents from a collection with the help of find() method. The query is as follows −> db.addFieldDemo.find().pretty();The following is the output −{    "_id" : ObjectId("5c7f654d8d10a061296a3c44"),    "EmployeeId" : 101,    "EmployeeName" : "Larry",    "EmployeeDetails" : {       "EmployeeSalary" : 65000,     ... Read More

To auto increment in MongoDB to store a sequence of unique user id, let us create a collection which contains information about last sequence values of all documents.Let us first create a collection. The query to create a collection which is as follows −> db.createSequenceDemo.insertOne({_id:"SID", S_Value:0}); { "acknowledged" : true, "insertedId" : "SID" }Now, we will create a function that will generate an auto increment in MongoDB to store sequence. The query is as follows −> function nextSequence(s) {    ... var sd = db.createSequenceDemo.findAndModify({       ... query:{_id: s },       ... update: {$inc:{S_Value:1}},     ... Read More

Use the aggregate framework to find all duplicate documents in a MongoDB collection by a key field.To understand the concept, let us create a collection with the document. The query to create a collection with a document is as follows −> db.findDuplicateByKeyDemo.insertOne({"StudentId":1, "StudentName":"John"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f5b168d10a061296a3c3a") } > db.findDuplicateByKeyDemo.insertOne({"StudentId":2, "StudentName":"Carol"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f5b1f8d10a061296a3c3b") } > db.findDuplicateByKeyDemo.insertOne({"StudentId":3, "StudentName":"Carol"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f5b248d10a061296a3c3c") } > db.findDuplicateByKeyDemo.insertOne({"StudentId":4, "StudentName":"John"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c7f5b2d8d10a061296a3c3d") } > db.findDuplicateByKeyDemo.insertOne({"StudentId":5, "StudentName":"Sam"}); {    "acknowledged" : true, ... Read More