It is saved in the special system.js collection. For this, use db.system.js.save(). Following is the syntax −db.system.js.save({    _id: "anyFunctionName",    value: function (returnValue) {       return ‘yourMessage ' + returnValue;    } })Let us implement the above syntax. Following is the query −> db.system.js.save({ ...    _id: "returnValue", ...    value: function (data) { ...       return 'The value==== ' + data; ...    } ... }) WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })Following is the query to call the above function to print the actual data −> db.eval("returnValue(20)") WARNING: db.eval is deprecatedThis will produce the following output −The value==== 20

To count number of documents, use count() in MongoDB. Let us create a collection with documents −> db.demo664.insertOne({_id:1, ClientName:"Chris"}); { "acknowledged" : true, "insertedId" : 1 } > db.demo664.insertOne({_id:2, ClientName:"Bob"}); { "acknowledged" : true, "insertedId" : 2 } > db.demo664.insertOne({_id:3, ClientName:"Sam"}); { "acknowledged" : true, "insertedId" : 3 } > db.demo664.insertOne({_id:4, ClientName:"David"}); { "acknowledged" : true, "insertedId" : 4 }Display all documents from a collection with the help of find() method −> db.demo664.find();This will produce the following output −{ "_id" : 1, "ClientName" : "Chris" } { "_id" : 2, "ClientName" : "Bob" } { "_id" : 3, "ClientName" : ... Read More

For bulk insert in MongoDB, use initializeUnorderedBulkOp(). Let us create a collection with documents −> var bulkInsertDoc = db.demo663.initializeUnorderedBulkOp(); > bulkInsertDoc.insert( { Name: "John", CountryName:"US"} ); > bulkInsertDoc.insert( { Name: "Chris", CountryName:"UK"} ); > bulkInsertDoc.insert( { Name: "David", CountryName:"AUS"} ); > bulkInsertDoc.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.demo663.find();This will produce the following output −{ "_id" : ... Read More

To filter out sub documents, use MongoDB aggregate and in that, use $unwind. Let us create a collection with documents −> db.demo662.insertOne( ... { ...    "details":[ ...   { ...       Name:"Chris", ...       Marks:35 ...    }, ...    { ...       Name:"Bob", ...       Marks:45 ...    }, ...    { ...       Name:"David", ...       Marks:30 ...    } ... ] ... } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea1b2be24113ea5458c7d04") }Display all documents from a collection with the help of ... Read More

For faster search, create index. For this, use createIndex(). Let us create a collection with documents −> db.demo661.createIndex({ListOfName:1}); {    "createdCollectionAutomatically" : true,    "numIndexesBefore" : 1,    "numIndexesAfter" : 2,    "ok" : 1 } > db.demo661.insertOne({_id:1, ListOfName:["John", "Robert", "David"]}); { "acknowledged" : true, "insertedId" : 1 } > db.demo661.insertOne({_id:2, ListOfName:["Mike", "Sam"]}); { "acknowledged" : true, "insertedId" : 2 } > db.demo661.insertOne({_id:3, ListOfName:["John", "David", "Bob"]}); { "acknowledged" : true, "insertedId" : 3 }Display all documents from a collection with the help of find() method −> db.demo661.find();This will produce the following output −{ "_id" : 1, "ListOfName" : [ "John", ... Read More

You can use $group. Let us create a collection with documents −> db.demo659.insertOne({Name:"Chris", CountryName:"US", "Marks":50}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea1a50724113ea5458c7cf9") } > db.demo659.insertOne({Name:"David", CountryName:"US", "Marks":60}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea1a50724113ea5458c7cfa") } > db.demo659.insertOne({Name:"Mike", CountryName:"US", "Marks":55}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea1a50724113ea5458c7cfb") } > db.demo659.insertOne({Name:"Chris", CountryName:"UK", "Marks":75}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea1a50724113ea5458c7cfc") } > db.demo659.insertOne({Name:"David", CountryName:"UK", "Marks":54}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea1a50724113ea5458c7cfd") } > db.demo659.insertOne({Name:"Mike", CountryName:"UK", "Marks":72}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea1a50824113ea5458c7cfe") }Display all documents from a collection ... Read More

For unique index, set unique − true while creating an index. Let us create a collection with documents −> db.demo658.createIndex({FirstName:1}, {unique:true, sparse:true}); {    "createdCollectionAutomatically" : true,    "numIndexesBefore" : 1,    "numIndexesAfter" : 2,    "ok" : 1 } > > db.demo658.insertOne({"FirstName":"John", "LastName":"Smith"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea067144deddd72997713d9") } > db.demo658.insertOne({"FirstName":"Adam", "LastName":"Smith"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea0671c4deddd72997713da") } > db.demo658.insertOne({"FirstName":"John", "LastName":"Doe"}); 2020-04-22T21:17:46.072+0530 E QUERY [js] WriteError: E11000 duplicate key error collection: onlinecustomertracker.demo658 index: FirstName_1 dup key: { : "John" } : WriteError({    "index" : 0,    "code" : 11000,   ... Read More

To group by dates, use $group in MongoDB aggregate. Let us create a collection with documents −> db.demo657.insertOne({ ...       id: 1, ...       Name: "Chris", ...       DueDate: new ISODate("2020-04-22") ...    } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea064b44deddd72997713d6") } > db.demo657.insertOne( ... { ...    id: 1, ...    Name: "John", ...    DueDate: new ISODate("2020-04-22") ... } ...); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea064b44deddd72997713d7") } > db.demo657.insertOne( ... { ...    id: 1, ...    Name: "Chris", ...    DueDate: new ISODate("2020-04-22") ... ... Read More

To push and slice in MongoDB, use $push and $slice. Let us create a collection with documents −> db.demo656.insertOne({Name:"John"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea060264deddd72997713cf") }Display all documents from a collection with the help of find() method −> db.demo656.find();This will produce the following output −{ "_id" : ObjectId("5ea060264deddd72997713cf"), "Name" : "John" }Here is the query to push and slice in MongoDB−> db.demo656.update({Name:"John"}, {"$push":{"ListOfName": {"$each": ["John"], "$slice": -9}}}); WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })Display all documents from a collection with the help of find() method −> db.demo656.find();This will produce the following output −{ ... Read More

To clear, use dropDatabase. Following is the syntax −use yourDatabaseName; db.dropDatabase();To clear a MongoDB database, first show all the databases −> show dbsThis will produce the following output −MyDB    0.000GB admin    0.000GB config    0.000GB local    0.000GB onlinecustomertracker 0.000GB test    0.006GBNow, let us delete the database onlinecustomertracker −> use onlinecustomertracker switched to db onlinecustomertracker > db.dropDatabase(); { "dropped" : "onlinecustomertracker", "ok" : 1 }Following is the query to show all databases after deleting a database above −> show dbsThis will produce the following output −MyDB    0.000GB admin    0.000GB config    0.000GB local    0.000GB test    0.006GB