To merge, use $setUnion operator. Let us first create a collection with documents −> db.mergeTwoArrayFieldDemo.insertOne({"NaturalNumbers":[1,2,3,8,10,20,30],"WholeNumbers":[0,1,2,63,78,20,45]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd68e4057806ebf1256f11d") }Following is the query to display all documents from a collection with the help of find() method −> db.mergeTwoArrayFieldDemo.find().pretty();This will produce the following output −{    "_id" : ObjectId("5cd68e4057806ebf1256f11d"),    "NaturalNumbers" : [       1,       2,       3,       8,       10,       20,       30    ],    "WholeNumbers" : [       0,       1,       2,       63,       78,       20,       45    ] }Following is the query to merge two array field in MongoDB.> db.mergeTwoArrayFieldDemo.aggregate([    { "$project": {       "MergedArray": { "$setUnion": [ "$NaturalNumbers", "$WholeNumbers" ] }    }} ]);This will produce the following output −{ "_id" : ObjectId("5cd68e4057806ebf1256f11d"), "MergedArray" : [ 0, 1, 2, 3, 8, 10, 20, 30, 45, 63, 78 ] }

Let us first create a collection with documents −> db.findByObjectIdDemo.insertOne({"ClientName":"Larry", "ClientAge":23}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd68cd657806ebf1256f11a") } > db.findByObjectIdDemo.insertOne({"ClientName":"Chris", "ClientAge":26}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd68cdc57806ebf1256f11b") } > db.findByObjectIdDemo.insertOne({"ClientName":"David", "ClientAge":38, "isMarried":true}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd68cf657806ebf1256f11c") }Following is the query to display all documents from a collection with the help of find() method −> db.findByObjectIdDemo.find().pretty();This will produce the following output −{    "_id" : ObjectId("5cd68cd657806ebf1256f11a"),    "ClientName" : "Larry",    "ClientAge" : 23 } {    "_id" : ObjectId("5cd68cdc57806ebf1256f11b"),    "ClientName" : "Chris",    "ClientAge" : 26 } { ... Read More

Let’s say you have saved the Login date of users. Now, you want the users who logged in between specific dates i.e. login date. For this, use $gte and $lt operator along with count(). Let us first create a collection with documents −> db.findDataByDateDemo.insertOne({"UserName":"John", "UserLoginDate":new ISODate("2019-01-31")}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cdd8cd7bf3115999ed511ed") } > db.findDataByDateDemo.insertOne({"UserName":"Larry", "UserLoginDate":new ISODate("2019-02-01")}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cdd8ce7bf3115999ed511ee") } > db.findDataByDateDemo.insertOne({"UserName":"Sam", "UserLoginDate":new ISODate("2019-05-02")}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cdd8cf3bf3115999ed511ef") } > db.findDataByDateDemo.insertOne({"UserName":"David", "UserLoginDate":new ISODate("2019-05-16")}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cdd8d00bf3115999ed511f0") } > db.findDataByDateDemo.insertOne({"UserName":"Carol", ... Read More

You can use $group operator with _id: null. Following is the syntax −db.yourCollectionName.aggregate([{$group: {_id:null, "anyFieldName": {$avg:"$yourFieldName"} } }]);Let us first create a collection with documents −> db.caculateTheAverageValueDemo.insertOne({"Population":100}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd68a197924bb85b3f4895f") } > db.caculateTheAverageValueDemo.insertOne({"Population":500}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd68a1c7924bb85b3f48960") } > db.caculateTheAverageValueDemo.insertOne({"Population":200}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd68a237924bb85b3f48961") } > db.caculateTheAverageValueDemo.insertOne({"Population":100}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd68a297924bb85b3f48962") } > db.caculateTheAverageValueDemo.insertOne({"Population":100}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd68a2e7924bb85b3f48963") }Following is the query to display all documents from a collection with the help of find() ... Read More

Use aggregate framework along with $ifNull operator for this. The $concatArrays in aggregation is used to concatenate arrays. Let us first create a collection with documents −>db.concatenateArraysDemo.insertOne({"FirstSemesterSubjects": ["MongoDB", "MySQL", "Java"], "SecondSemesterSubjects":["C", "C++", ]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd687707924bb85b3f4895c") } > db.concatenateArraysDemo.insertOne({"FirstSemesterSubjects":["C#", "Ruby", "Python"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd687927924bb85b3f4895d") } >db.concatenateArraysDemo.insertOne({"FirstSemesterSubjects":["HTML", "CSS", "Javascript"], "SecondSemesterSubjects":["CSS", "Javascript"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd687bb7924bb85b3f4895e") }Following is the query to display all documents from a collection with the help of find() method −> db.concatenateArraysDemo.find().pretty();This will produce the following output −{    "_id" : ObjectId("5cd687707924bb85b3f4895c"), ... Read More

You can use $in operator to check if a value is in an array or not. Let us first create a collection with documents −> db.valueInArrayDemo.insertOne({"UserName":"John", "UserMessage":["Hi", "Hello", "Bye"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd684cf7924bb85b3f48959") } > db.valueInArrayDemo.insertOne({"UserName":"Larry", "UserMessage":["Thank You", "Amazing", "Nice"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd684d27924bb85b3f4895a") } >db.valueInArrayDemo.insertOne({"UserName":"Carol", "UserMessage":["Awesome", "Bye", "Cool"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd684d87924bb85b3f4895b") }Following is the query to display all documents from a collection with the help of find() method −> db.valueInArrayDemo.find().pretty();This will produce the following output −{    "_id" : ObjectId("5cd684cf7924bb85b3f48959"),    "UserName" : "John",    "UserMessage" : [       "Hi",       "Hello",     ... Read More

To push elements to an existing array, use $addToSet operator along with update(). Let us first create a collection with documents −> db.pushElements.insertOne({"Comments":["Good", "Awesome", "Nice"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd682597924bb85b3f48953") }Following is the query to display all documents from a collection with the help of find() method −> db.pushElements.find().pretty();This will produce the following output −{    "_id" : ObjectId("5cd682597924bb85b3f48953"),    "Comments" : [       "Good",       "Awesome",       "Nice"    ] }Following is the query to push elements to an existing array in MongoDB −> db.pushElements.update(    {_id:ObjectId("5cd682597924bb85b3f48953")},    { ... Read More

For this, work with the concept of forEach(). Let us first create a collection with documents −> db.printDocuementValueDemo.insertOne({"InstructorName":"John Smith"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd6804f7924bb85b3f48950") } > db.printDocuementValueDemo.insertOne({"InstructorName":"Sam Williams"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd680577924bb85b3f48951") } > db.printDocuementValueDemo.insertOne({"InstructorName":"David Miller"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd680637924bb85b3f48952") }Following is the query to display all documents from a collection with the help of find() method −> db.printDocuementValueDemo.find().pretty();This will produce the following output −{    "_id" : ObjectId("5cd6804f7924bb85b3f48950"),    "InstructorName" : "John Smith" } {    "_id" : ObjectId("5cd680577924bb85b3f48951"),    "InstructorName" : "Sam Williams" } ... Read More

For this, you need to write some code using forEach(). Let us first create a collection with documents −> db.removingWhiteSpaceDemo.insertOne({"Title":" Introduction to java "}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd66f387924bb85b3f4894c") }Following is the query to display all documents from a collection with the help of find() method −> db.removingWhiteSpaceDemo.find();This will produce the following output −{ "_id" : ObjectId("5cd66f387924bb85b3f4894c"), "Title" : " Introduction to java " }Following is the query to remove white spaces (leading and trailing) from string value −> db.removingWhiteSpaceDemo.find({}, {"Title": 1 }).forEach(function(myDocument) {    myDocument.Title = myDocument.Title.trim();    db.removingWhiteSpaceDemo.update(       { "_id": myDocument._id ... Read More

You can use $literal operator along with aggregate framework. Let us first create a collection with documents −> db.fieldWithStaticValue.insertOne({"Name":"Larry", "Age":24}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd6554c7924bb85b3f48948") } > db.fieldWithStaticValue.insertOne({"Name":"Chris", "Age":23}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd655567924bb85b3f48949") } > db.fieldWithStaticValue.insertOne({"Name":"David", "Age":26}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd655607924bb85b3f4894a") }Following is the query to display all documents from a collection with the help of find() method −> db.fieldWithStaticValue.find();This will produce the following output −{ "_id" : ObjectId("5cd6554c7924bb85b3f48948"), "Name" : "Larry", "Age" : 24 } { "_id" : ObjectId("5cd655567924bb85b3f48949"), "Name" : "Chris", "Age" : 23 ... Read More