To reduce MongoDB storage space after deleting a large amount of data, you need to use repairDatabase(). Let’s say we are using the database “test”. Following is the query to get to the current database> db;This will produce the following outputTestAfter deleting large amount of data, this is how you can reduce the storage space of MongoDB> db.repairDatabase()This will produce the following output{ "ok" : 1 }Here is the screenshot after we implement the above query. This will reduce the storage space:

To concatenate strings from two fields into a third field, you can use the following syntaxdb.yourCollectionName.aggregate(    [       { $project: { "yourNewFieldName": { $concat: [ "$yourFieldName1", " yourDellimiterValue ", "$yourFieldName2" ] } } }    ] );Let us first create a collection with documents>db.concatenateStringsDemo.insertOne({"StudentFirstName":"John", "StudentLastName":"Doe"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9bb7362d66697741252444") } >db.concatenateStringsDemo.insertOne({"StudentFirstName":"John", "StudentLastName":"Smith"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9bb7402d66697741252445") } >db.concatenateStringsDemo.insertOne({"StudentFirstName":"Carol", "StudentLastName":"Taylor"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9bb74c2d66697741252446") } >db.concatenateStringsDemo.insertOne({"StudentFirstName":"David", "StudentLastName":"Miller"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9bb7752d66697741252447") } >db.concatenateStringsDemo.insertOne({"StudentFirstName":"James", "StudentLastName":"Williams"}); {    "acknowledged" : ... Read More

To remove _id from MongoDB result, you need to set 0 for _id field. Following is the syntaxdb.yourCollectionName.find({}, {_id:0});To understand it, let us create a collection with documents. Following is the query> db.removeIdDemo.insertOne({"UserName":"John", "UserAge":23}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9bb4042d66697741252440") } > db.removeIdDemo.insertOne({"UserName":"Mike", "UserAge":27}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9bb40c2d66697741252441") } > db.removeIdDemo.insertOne({"UserName":"Sam", "UserAge":34}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9bb4162d66697741252442") } > db.removeIdDemo.insertOne({"UserName":"Carol", "UserAge":29}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9bb4222d66697741252443") }Following is the query to display all documents from a collection with the help of find() method> db.removeIdDemo.find().pretty();This ... Read More

You need to use update command along with $pull operator to delete documents within an array. Let us create a collection with documents. Following is the query> db.deleteDocumentsDemo.insertOne( ... { ...    "_id":100, ...    "StudentsDetails" : [ ...       { ...          "StudentId" : 1, ...          "StudentName" : "John" ...       }, ...       { ...          "StudentId" : 2, ...          "StudentName" : "Carol" ...       }, ...       { ...         ... Read More

To change the password in MongoDB for existing user, you can use changeUserPassword(). Following is the syntaxdb.changeUserPassword("yourExistingUserName", "yourPassword");Let us first switch the database to admin. Following is the syntax> use adminThis will produce the following outputswitched to db adminNow, display users from the database. Following is the query> db.getUsers();This will produce the following output[    {       "_id" : "admin.John",       "user" : "John",       "db" : "admin",       "roles" : [          {             "role" : "userAdminAnyDatabase",             "db" : "admin"          }       ],       "mechanisms" : [          "SCRAM-SHA-1",          "SCRAM-SHA-256"       ]    } ]Following is the query to change the password for user “John”> db.changeUserPassword("John", "123456");Now the password has been changed with value “123456”.

To operate on all databases from MongoDB shell, you can use listDatabases along with adminCommand().Let’s say we are using a sample database “test”. At first, check the current database with the help of db command.Following is the query to get the current database> db;This will produce the following outputTestFollowing is the query to operate on all the databases from the Mongo shell> var allDatabaseList = db.adminCommand('listDatabases');Now you need to use printjson() in order to print all databases. Following is the query> printjson (allDatabaseList);This will produce the following output{    "databases" : [       {         ... Read More

Let us first create a collection with documents> db.updateListOfKeyValuesDemo.insertOne( { "StudentDetails":[ { "StudentName":"John", "StudentAge":23, "StudentCountryName":"US" }, { "StudentName":"Carol", "StudentAge":24, "StudentCountryName":"UK" }, { "StudentName":"Bob", "StudentAge":22, "StudentCountryName":"AUS" } ] } ); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b5b759882024390176545") }Following is the query to display all documents from a collection with the help of find() method> db.updateListOfKeyValuesDemo.find().pretty();This will produce the following output{    "_id" : ObjectId("5c9b5b759882024390176545"),    "StudentDetails" : [       {          "StudentName" : "John",          "StudentAge" : 23,          "StudentCountryName" : "US"       },     ... Read More

Yes, it is possible using the $project operator. Let us first create a collection with documents> db.sumTwoFieldsDemo.insertOne({"FirstValue":150, "SecondValue":350}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b4bfe15e86fd1496b38cd") } > db.sumTwoFieldsDemo.insertOne({"FirstValue":450, "SecondValue":1550}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b4c1215e86fd1496b38ce") } > db.sumTwoFieldsDemo.insertOne({"FirstValue":2560, "SecondValue":2440}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b4c2715e86fd1496b38cf") }Following is the query to display all documents from a collection with the help of find() method> db.sumTwoFieldsDemo.find().pretty();This will produce the following output{    "_id" : ObjectId("5c9b4bfe15e86fd1496b38cd"),    "FirstValue" : 150,    "SecondValue" : 350 } {    "_id" : ObjectId("5c9b4c1215e86fd1496b38ce"),    "FirstValue" : 450,    "SecondValue" ... Read More

Following is the query to get distinct values with sorted data in MongoDBdb.yourCollectionName.distinct("yourFieldName").sort();Let us first create a collection with documents>db.getDistinctWithSortedDataDemo.insertOne({"StudentId":10, "StudentName":"John", "StudentAge":23}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b1e3315e86fd1496b38c3") } >db.getDistinctWithSortedDataDemo.insertOne({"StudentId":20, "StudentName":"Carol", "StudentAge":21}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b1e3e15e86fd1496b38c4") } >db.getDistinctWithSortedDataDemo.insertOne({"StudentId":10, "StudentName":"John", "StudentAge":23}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b1e4415e86fd1496b38c5") } >db.getDistinctWithSortedDataDemo.insertOne({"StudentId":30, "StudentName":"Chris", "StudentAge":22}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b1e5115e86fd1496b38c6") } >db.getDistinctWithSortedDataDemo.insertOne({"StudentId":20, "StudentName":"Carol", "StudentAge":21}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b1e5715e86fd1496b38c7") } >db.getDistinctWithSortedDataDemo.insertOne({"StudentId":40, "StudentName":"Bob", "StudentAge":20}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b1e6515e86fd1496b38c8") } >db.getDistinctWithSortedDataDemo.insertOne({"StudentId":40, "StudentName":"Bob", "Stude ... Read More

There is a $toLower operator in MongoDB to be used as part of aggregate framework. But, we can also use the for loop to iterate over the specific field and update one by one.Let us first create a collection with documents> db.toLowerDemo.insertOne({"StudentId":101, "StudentName":"John"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b1b4515e86fd1496b38bf") } > db.toLowerDemo.insertOne({"StudentId":102, "StudentName":"Larry"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b1b4b15e86fd1496b38c0") } > db.toLowerDemo.insertOne({"StudentId":103, "StudentName":"CHris"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b1b5115e86fd1496b38c1") } > db.toLowerDemo.insertOne({"StudentId":104, "StudentName":"ROBERT"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9b1b5a15e86fd1496b38c2") }Following is the query to display all documents from ... Read More