For this, we will use the concept of createIndex() and create index −> db.compoundIndexDemo.createIndex({"StudentName":1, "StudentAge":1}, {unique:true}); {    "createdCollectionAutomatically" : true,    "numIndexesBefore" : 1,    "numIndexesAfter" : 2,    "ok" : 1 }Let us first create a collection with documents −> db.compoundIndexDemo.insertOne({"StudentName":"Chris"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e084c1d25ddae1f53b62207") } > db.compoundIndexDemo.insertOne({"StudentName":"Chris", "StudentAge":23}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e084c5625ddae1f53b62209") } > db.compoundIndexDemo.insertOne({"StudentName":"Chris", "StudentAge":22}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e084c5b25ddae1f53b6220a") } > db.compoundIndexDemo.insertOne({"StudentName":"Chris", "StudentAge":23}); 2019-12-29T12:19:02.225+0530 E QUERY [js] WriteError: E11000 duplicate key error collection: web.compoundIndexDemo index: StudentName_1_StudentAge_1 dup key: { : "Chris", ... Read More

You cannot update a column value using itself. For this, you can use $set. Let us create a collection with documents −> db.demo256.insertOne({"Name":"Chris"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e47a3e91627c0c63e7dba8b") } > db.demo256.insertOne({"Name":"Bob"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e47a3ea1627c0c63e7dba8c") } > db.demo256.insertOne({"Name":"David"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e47a3eb1627c0c63e7dba8d") }Display all documents from a collection with the help of find() method −> db.demo256.find();This will produce the following output −{ "_id" : ObjectId("5e47a3e91627c0c63e7dba8b"), "Name" : "Chris" } { "_id" : ObjectId("5e47a3ea1627c0c63e7dba8c"), "Name" : "Bob" } { "_id" : ObjectId("5e47a3eb1627c0c63e7dba8d"), "Name" : "David" }Following is ... Read More

For bulk insert, you can use insert() in MongoDB. Let us create a collection with documents −> var manyDocument = db.demo255.initializeUnorderedBulkOp(); > manyDocument.insert( { "Name":"Chris", Age:24} ); > manyDocument.insert( {"Name":"Bob", Age:22 } ); > manyDocument.insert( { "Name":"David", Age:23 } ); > manyDocument.execute(); BulkWriteResult({    "writeErrors" : [ ],    "writeConcernErrors" : [ ],    "nInserted" : 3,    "nUpserted" : 0,    "nMatched" : 0,    "nModified" : 0,    "nRemoved" : 0,    "upserted" : [ ] })Display all documents from a collection with the help of find() method −> db.demo255.find();This will produce the following output −{ "_id" : ... Read More

For this, use mapReduce(). Let us first create a collection with documents −> db.splitString.insertOne({"StudentName":"John Smith"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e0849d925ddae1f53b62206") }Following is the query to display all documents from a collection with the help of find() method −> db.splitString.find().pretty();This will produce the following output −{    "_id" : ObjectId("5e0849d925ddae1f53b62206"),    "StudentName" : "John Smith" }Here is the query to split a string −> db.splitString.mapReduce( ...    function() { ...       var StudentLastName = this.StudentName.split(/\s/).reverse()[0].toUpperCase(); ... ...       emit({ "StudentLastName": StudentLastName, "FirstObjectId": this._id }, this); ...    }, ...    function(){}, ...   ... Read More

Fir this, you can use $rename. Let us first create a collection with documents −> db.changeSimpleFieldDemo.insertOne({"StudentMarks":58, "StudentSubject":"MySQL"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e0847a825ddae1f53b62205") }Following is the query to display all documents from a collection with the help of find() method −> db.changeSimpleFieldDemo.find();This will produce the following output −{ "_id" : ObjectId("5e0847a825ddae1f53b62205"), "StudentMarks" : 58, "StudentSubject" : "MySQL" }Here is the query to change a field into an object. The “obj” field is a temporary field name we have used below −> db.changeSimpleFieldDemo.update({}, {$rename: {Student: 'obj'}}, {multi: true}); WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 0 ... Read More

To sort in MongoDB, use sort(). For displaying only a specific number of values, use LIMIT. Let us create a collection with documents −> db.demo254.insertOne({"Name":"Chris"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e47a0ab1627c0c63e7dba7f") } > db.demo254.insertOne({"Name":"Adam"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e47a0b01627c0c63e7dba80") } > db.demo254.insertOne({"Name":"Bob"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e47a0b71627c0c63e7dba81") }Display all documents from a collection with the help of find() method −> db.demo254.find();This will produce the following output −{ "_id" : ObjectId("5e47a0ab1627c0c63e7dba7f"), "Name" : "Chris" } { "_id" : ObjectId("5e47a0b01627c0c63e7dba80"), "Name" : "Adam" } { "_id" : ObjectId("5e47a0b71627c0c63e7dba81"), "Name" : "Bob" ... Read More

To display number of databases in MongoDB, you need to create atleast one document in a database.Let’s say, you have created a database, but did not added any document in it. Then in the list of databases that particular database won’t be visible.Following is the query to create a database −> use app; switched to db appFollowing is the query to display all databases −> show dbs;This will produce the following output. The new database “app” won’t be visible since we haven’t added atleast one document in it −admin                0.002GB business     ... Read More

To match and remove element(s) , use MongoDB $pullAll. Let us first create a collection with documents −> db.removeElementsDemo.insertOne({"ListOfNames":["Mike", "Sam", "David", "Carol"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e071e5a25ddae1f53b62203") }Following is the query to display all documents from a collection with the help of find() method −> db.removeElementsDemo.find().pretty();This will produce the following output −{    "_id" : ObjectId("5e071e5a25ddae1f53b62203"),       "ListOfNames" : [          "Mike",          "Sam",          "David",          "Carol"    ] }Here is the query to match and remove element(s) from an array ... Read More

To find by id in MongoDB, use the find() method as in the below syntax −db.findByIdDemo.find({"_id" :yourObjectId});To understand the above syntax, let us create a collection with documents −> db.findByIdDemo.insertOne({"Value":10}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e07158925ddae1f53b621fc") } > db.findByIdDemo.insertOne({"Value":500}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e07158c25ddae1f53b621fd") } > db.findByIdDemo.insertOne({"Value":1000}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e07159125ddae1f53b621fe") }Following is the query to display all documents from a collection with the help of find() method −> db.findByIdDemo.find();This will produce the following output −"_id" : ObjectId("5e07158925ddae1f53b621fc"), "Value" : 10 } { "_id" : ObjectId("5e07158c25ddae1f53b621fd"), "Value" : ... Read More

Use find() along with $elemMatch to fetch array values. Let us first create a collection with documents −> db.fetchingArrayValuesDemo.insertOne( ... { ...    "StudentName": "David", ...    "StudentDetails": [ ...       { ...          "FatherName": "Bob", ...          "CountryName": "US", ... ...          "Favourite": [ ...             { ...                "Teacher": "DAVID", ...                "Subject": [ ...                   "MySQL", ...         ... Read More