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

To implement indexes correctly with $elemMatch, you need to use the concept of explain(). Let us first create a collection with documents −> db.workingOfIndexesDemo.createIndex({"Information.StudentDetails.StudentName":1}, { sparse : true, background : true } ); {    "createdCollectionAutomatically" : true,    "numIndexesBefore" : 1,    "numIndexesAfter" : 2,    "ok" : 1 } > db.workingOfIndexesDemo.insertOne({"Information":{"StudentDetails":{"StudentName":"Chris"}}}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e06f94825ddae1f53b621f7") } > db.workingOfIndexesDemo.insertOne({"Information":{"StudentDetails":{"StudentName":"David"}}}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e06f94f25ddae1f53b621f8") } > db.workingOfIndexesDemo.insertOne({"Information":{"StudentDetails":{"StudentName":"Mike"}}}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e06f95325ddae1f53b621f9") }Following is the query to display all documents from a collection with the help ... Read More

For this, you can use $cond along with $anyElementTrue. NULL values (absence of a field) would evaluate to FALSE. With that, an empty array also returns FALSE with $ anyElementTrue.Let us first create a collection with documents −> db.presenceDemo.insertOne({"StudentName":null}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e06f70c25ddae1f53b621f3") } > db.presenceDemo.insertOne({"StudentName":"Chris"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e06f71425ddae1f53b621f4") } > db.presenceDemo.insertOne({"StudentName":null}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e06f71825ddae1f53b621f5") } > db.presenceDemo.insertOne({"StudentName":"David"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e06f71e25ddae1f53b621f6") }Following is the query to display all documents from a collection with the help of find() ... Read More

For this, use double underscore( __) in find(). Let us first create a collection with documents −> db.nestedDemo.insertOne({"Information":{"__StudentName":"John Smith"}}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e06f39125ddae1f53b621f0") } > db.nestedDemo.insertOne({"Information":{"__StudentName":"John Doe"}}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e06f39e25ddae1f53b621f1") } > db.nestedDemo.insertOne({"Information":{"__StudentName":"Chris Brown"}}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e06f3a625ddae1f53b621f2") }Following is the query to display all documents from a collection with the help of find() method −> db.nestedDemo.find().pretty();This will produce the following output −{    "_id" : ObjectId("5e06f39125ddae1f53b621f0"),    "Information" : {       "__StudentName" : "John Smith"    } } {    "_id" ... Read More

For this, you can use aggregate(). Let us first create a collection with documents −> db.countOccurrencesDemo.insertOne({"ListOfValues":[65, 87, 89, 65, 67, 87, 87, 87]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e06ef9325ddae1f53b621eb") } > db.countOccurrencesDemo.insertOne({"ListOfValues":[102, 65, 87, 65, 89, 65, 89, 65, 89, 65]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e06efaa25ddae1f53b621ec") }Following is the query to display all documents from a collection with the help of find() method −> db.countOccurrencesDemo.find();This will produce the following output −{ "_id" : ObjectId("5e06ef9325ddae1f53b621eb"), "ListOfValues" : [ 65, 87, 89, 65, 67, 87, 87, 87 ] } { "_id" : ObjectId("5e06efaa25ddae1f53b621ec"), "ListOfValues" : ... Read More