How to create an index with MongoDB?

To create an index in MongoDB, use the ensureIndex() method. Let us first create a collection using following query

> db.createCollection(&qu/ot;creatingUniqueIndexDemo"); { "ok" : 1 }

Following is the query to create an index on the above collection:

> db.creatingUniqueIndexDemo.ensureIndex({"UserCountryName":1},{unique:true}); {    "createdCollectionAutomatically" : false,    "numIndexesBefore" : 1,    "numIndexesAfter" : 2,    "ok" : 1 }

Following is the query to insert some documents in the above collection

>db.creatingUniqueIndexDemo.insertOne({"UserName":"John","UserAge":21,"UserCountryName":"US"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9947bd330fd0aa0d2fe4d8") } >db.creatingUniqueIndexDemo.insertOne({"UserName":"Mike","UserAge":23,"UserCountryName":"UK"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9947c9330fd0aa0d2fe4d9") } >db.creatingUniqueIndexDemo.insertOne({"UserName":"Robert","UserAge":26,"UserCountryName":"US"}); 2019-03-26T02:57:52.670+0530 E QUERY [js] WriteError: E11000 duplicate key error collection: test.creatingUniqueIndexDemo index: UserCountryName_1 dup key: { : "US" } : WriteError({    "index" : 0,    "code" : 11000,    "errmsg" : "E11000 duplicate key error collection: test.creatingUniqueIndexDemo index: .      UserCountryName_1 dup key: { : \"US\" }",    "op" : {       "_id" : ObjectId("5c9947d8330fd0aa0d2fe4da"),       "UserName" : "Robert",       "UserAge" : 26,       "UserCountryName" : "US"    } }) WriteError@src/mongo/shell/bulk_api.js:461:48 Bulk/mergeBatchResults@src/mongo/shell/bulk_api.js:841:49 Bulk/executeBatch@src/mongo/shell/bulk_api.js:906:13 Bulk/this.execute@src/mongo/shell/bulk_api.js:1150:21 DBCollection.prototype.insertOne@src/mongo/shell/crud_api.js:252:9 @(shell):1:1 > db.creatingUniqueIndexDemo.insertOne({"UserName":"Robert","UserAge":26,"UserCountryName":"AUS"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9947e3330fd0aa0d2fe4db") }

Following is the query to display all documents from a collection with the help of find() method

> db.creatingUniqueIndexDemo.find().pretty();

This will produce the following output

{    "_id" : ObjectId("5c9947bd330fd0aa0d2fe4d8"),    "UserName" : "John",    "UserAge" : 21,    "UserCountryName" : "US" } {    "_id" : ObjectId("5c9947c9330fd0aa0d2fe4d9"),    "UserName" : "Mike",    "UserAge" : 23,    "UserCountryName" : "UK" } {    "_id" : ObjectId("5c9947e3330fd0aa0d2fe4db"),    "UserName" : "Robert",    "UserAge" : 26,    "UserCountryName" : "AUS" }