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

The findOne() returns one document that satisfies the specified query criteria on the collection. Let us create a collection with documents −> db.demo655.insertOne({subject:"MySQL"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea050254deddd72997713cc") } > db.demo655.insertOne({subject:"MongoDB"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea0502b4deddd72997713cd") } > db.demo655.insertOne({subject:"Java"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea050314deddd72997713ce") }Display all documents from a collection with the help of find() method −> db.demo655.find();This will produce the following output −{ "_id" : ObjectId("5ea050254deddd72997713cc"), "subject" : "MySQL" } { "_id" : ObjectId("5ea0502b4deddd72997713cd"), "subject" : "MongoDB" } { "_id" : ObjectId("5ea050314deddd72997713ce"), "subject" : "Java" }Following is ... Read More

Yes, using $in is faster. Let us see an example and create a collection with documents −> db.demo653.insertOne({subject:"MySQL"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea04b274deddd72997713c0") } > db.demo653.insertOne({subject:"MongoDB"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea04b304deddd72997713c1") } > db.demo653.insertOne({subject:"Java"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea04b354deddd72997713c2") } > db.demo653.insertOne({subject:"C"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea04b384deddd72997713c3") } > db.demo653.insertOne({subject:"C++"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea04b3b4deddd72997713c4") }Display all documents from a collection with the help of find() method −> db.demo653.find();This will produce the following output −{ "_id" : ObjectId("5ea04b274deddd72997713c0"), "subject" : ... Read More

Use $sort in MongoDB aggregation. Let us create a collection with documents −> db.demo652.insertOne({ ...    value:10, ...    "details" : [{ ...       "ProductName" : "Product-1", ...       "ProductQuantity" : 8, ...       "ProductPrice" : 500 ...    }, { ...          "ProductName" : "Product-2", ...          "ProductQuantity" : 7, ...          "ProductPrice" : 500 ... ...       }] ...    } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5e9f0730e3c3cd0dcff36a62") } > > db.demo652.insertOne({ ...    value:5, ...   ... Read More

To avoid performance issues in MongoDB, use the concept of index. Let us create a collection with documents −> db.demo531.createIndex({"CountryName":"text", "Name":"text"});{    "createdCollectionAutomatically" : true,    "numIndexesBefore" : 1,    "numIndexesAfter" : 2,    "ok" : 1 } > db.demo531.insertOne({CountryName:"US", "Name":"Chris"});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b2b60ef4dcbee04fbbbf2") } > db.demo531.insertOne({CountryName:"UK", "Name":"David"});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b2b6cef4dcbee04fbbbf3") } > db.demo531.insertOne({CountryName:"AUS", "Name":"chris"});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b2b74ef4dcbee04fbbbf4") } > db.demo531.insertOne({CountryName:"US", "Name":"CHRIS"});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b2badef4dcbee04fbbbf5") }Display all documents from a collection with the help of find() method −> db.demo531.find();This will ... Read More

If there are null values also, then implement ORDERBY using sort().Note − Since, starting in MongoDB v3.2, the $orderby operator deprecated in the mongo shell. Use cursor.sort() instead.Let us create a collection with documents −> db.demo530.insertOne({"Name":"Chris"});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b2990ef4dcbee04fbbbec") } > db.demo530.insertOne({"Name":null});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b2991ef4dcbee04fbbbed") } > db.demo530.insertOne({"Name":"David"});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b2992ef4dcbee04fbbbee") } > db.demo530.insertOne({"Name":"Adam"});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b2995ef4dcbee04fbbbef") } > db.demo530.insertOne({"Name":null});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b2999ef4dcbee04fbbbf0") } > db.demo530.insertOne({"Name":"Carol"});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b299eef4dcbee04fbbbf1") }Display ... Read More

To group by _id in MongoDB, use $group. Let us create a collection with documents −> db.demo529.insertOne({"Score":10});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b1d5bef4dcbee04fbbbe4") } > db.demo529.insertOne({"Score":20});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b1d5fef4dcbee04fbbbe5") } > db.demo529.insertOne({"Score":10});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b1d61ef4dcbee04fbbbe6") } > db.demo529.insertOne({"Score":10});{    "acknowledged" : true,    "insertedId" : ObjectId("5e8b1d62ef4dcbee04fbbbe7") }Display all documents from a collection with the help of find() method −> db.demo529.find();This will produce the following output −{ "_id" : ObjectId("5e8b1d5bef4dcbee04fbbbe4"), "Score" : 10 } { "_id" : ObjectId("5e8b1d5fef4dcbee04fbbbe5"), "Score" : 20 } { "_id" : ObjectId("5e8b1d61ef4dcbee04fbbbe6"), "Score" : 10 ... Read More