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To merge, use aggregate() along with $PUSH. Let us create a collection with documents −> db.demo64.insertOne({"Subjects":["MySQL", "MongoDB", "Java"]}); { "acknowledged" : true, "insertedId" : ObjectId("5e28762bcfb11e5c34d89936") } > db.demo64.insertOne({"Subjects":["Oracle", "Spring", "Python"]}); { "acknowledged" : true, "insertedId" : ObjectId("5e28763fcfb11e5c34d89937") }Display all documents from a collection with the help of find() method −> db.demo64.find();This will produce the following output −{ "_id" : ObjectId("5e28762bcfb11e5c34d89936"), "Subjects" : [ "MySQL", "MongoDB", "Java" ] } { "_id" : ObjectId("5e28763fcfb11e5c34d89937"), "Subjects" : [ "Oracle", "Spring", "Python" ] }Following is the query to merge array of document in MongoDB −> db.demo64.aggregate([ ... { "$group": { ... Read More
REPL stands for Read-Eval-Print-Loop. It is a shell where the user types an expression, it's evaluated, and the result returned to the user. The main purpose of using REPL is to interact quickly with Java programs without creating a java file, compile it, and run it. JShell is very useful for the developers and allows us to learn the Java language.Below are some of the features of REPLIt’s built-in in Java 9.We can test any Java expression without a class file, compiling and running it.It autocompletes the methods, just typing the TAB key, as in your editor.We can define methods, and ... Read More
The Optional class provides a container that may or may not contain a non-null value. It has been introduced in Java 8 to reduce the number of places in the code where a NullPointerException has generated. Java 9 added three methods: ifPresentOrElse(), or(), and stream(), which helps us deal with default values.In the below example, we can get a stream from Optional class using Person classExampleimport java.util.Optional; import java.util.stream.Stream; public class OptionalTest { public static void main(String args[]) { getPerson().stream() .map(Person::getName) ... Read More
You need to use $group to group documents with specified _id expression. Let us first create a collection with documents −> db.aggreagateDemo.insertOne({"Product_Id":1, "ProductPrice":50}); { "acknowledged" : true, "insertedId" : ObjectId("5e06d3c025ddae1f53b621d9") } > db.aggreagateDemo.insertOne({"Product_Id":2, "ProductPrice":100}); { "acknowledged" : true, "insertedId" : ObjectId("5e06d3c625ddae1f53b621da") } > db.aggreagateDemo.insertOne({"Product_Id":2, "ProductPrice":500}); { "acknowledged" : true, "insertedId" : ObjectId("5e06d3cb25ddae1f53b621db") } > db.aggreagateDemo.insertOne({"Product_Id":1, "ProductPrice":150}); { "acknowledged" : true, "insertedId" : ObjectId("5e06d3d125ddae1f53b621dc") }Following is the query to display all documents from a collection with the help of find() method −> db.aggreagateDemo.find().pretty();This will produce the following output −{ "_id" : ObjectId("5e06d3c025ddae1f53b621d9"), ... Read More
To check for existence of a record, use findOne() in MongoDB. Let us first create a collection with documents −> db.existsAlternateDemo.insertOne({"StudentName":"Chris"}); { "acknowledged" : true, "insertedId" : ObjectId("5e06d23f9e4dae213890ac5c") } > db.existsAlternateDemo.insertOne({"StudentName":"Chris", "StudentAge":21}); { "acknowledged" : true, "insertedId" : ObjectId("5e06d2559e4dae213890ac5d") } >db.existsAlternateDemo.insertOne({"StudentName":"Chris", "StudentAge":22, "StudentCountryName":"US"}); { "acknowledged" : true, "insertedId" : ObjectId("5e06d2689e4dae213890ac5e") }Following is the query to display all documents from a collection with the help of find() method −> db.existsAlternateDemo.find();This will produce the following output −{ "_id" : ObjectId("5e06d23f9e4dae213890ac5c"), "StudentName" : "Chris" } { "_id" : ObjectId("5e06d2559e4dae213890ac5d"), "StudentName" : "Chris", "StudentAge" : 21 } { ... Read More
To sum specific fields, use aggregate along with $sum. Let us first create a collection with documents −> db.getSumOfFieldsDemo.insertOne({"Customer_Id":101, "Price":50, "Status":"Active"}); { "acknowledged" : true, "insertedId" : ObjectId("5e06cec29e4dae213890ac55") } > db.getSumOfFieldsDemo.insertOne({"Customer_Id":102, "Price":200, "Status":"Inactive"}); { "acknowledged" : true, "insertedId" : ObjectId("5e06ced19e4dae213890ac56") } > db.getSumOfFieldsDemo.insertOne({"Customer_Id":101, "Price":3000, "Status":"Active"}); { "acknowledged" : true, "insertedId" : ObjectId("5e06cedd9e4dae213890ac57") } > db.getSumOfFieldsDemo.insertOne({"Customer_Id":103, "Price":400, "Status":"Active"}); { "acknowledged" : true, "insertedId" : ObjectId("5e06cee79e4dae213890ac58") }Following is the query to display all documents from a collection with the help of find() method −> db.getSumOfFieldsDemo.find().pretty();This will produce the following output −{ "_id" : ... Read More
To insert records in MongoDB and avoid duplicates, use “unique:true”. Let us first create a collection with documents.Here, we are trying to add duplicate records −> db.insertWithoutDuplicateDemo.createIndex({"StudentFirstName":1}, { unique: true } ); { "createdCollectionAutomatically" : true, "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 } > db.insertWithoutDuplicateDemo.insert({"StudentFirstName":"Chris"}, { upsert: true }); WriteResult({ "nInserted" : 1 }) > db.insertWithoutDuplicateDemo.insert({"StudentFirstName":"David"}, { upsert: true }); WriteResult({ "nInserted" : 1 }) > db.insertWithoutDuplicateDemo.insert({"StudentFirstName":"Chris"}, { upsert: true }); WriteResult({ "nInserted" : 0, "writeError" : { "code" : 11000, "errmsg" : "E11000 duplicate ... Read More
To apply a condition, use $setIsSubset. Let us first create a collection with documents −> db.subsetDemo.insertOne({"StudentName":"Chris", "StudentFavouriteSubject":["Java", "Python"]}); { "acknowledged" : true, "insertedId" : ObjectId("5e063e49150ee0e76c06a052") } > db.subsetDemo.insertOne({"StudentName":"Chris", "StudentFavouriteSubject":["Java", "Python", "MySQL"]}); { "acknowledged" : true, "insertedId" : ObjectId("5e063e4f150ee0e76c06a053") }Following is the query to display all documents from a collection with the help of find() method −> db.subsetDemo.find().pretty();This will produce the following output −{ "_id" : ObjectId("5e063e49150ee0e76c06a052"), "StudentName" : "Chris", "StudentFavouriteSubject" : [ "Java", "Python" ] } { "_id" : ObjectId("5e063e4f150ee0e76c06a053"), "StudentName" : "Chris", ... Read More
At first, get the current month and subtract by 1 to fetch previous month records. Let us first create a collection with documents −> db.findOneMonthAgoData.insertOne({"CustomerName":"Chris", "PurchaseDate":new ISODate("2019-12-26")}); { "acknowledged" : true, "insertedId" : ObjectId("5e04e16c150ee0e76c06a04f") } > db.findOneMonthAgoData.insertOne({"CustomerName":"David", "PurchaseDate":new ISODate("2019-11-26")}); { "acknowledged" : true, "insertedId" : ObjectId("5e04e178150ee0e76c06a050") } > db.findOneMonthAgoData.insertOne({"CustomerName":"Bob", "PurchaseDate":new ISODate("2020-11-26")}); { "acknowledged" : true, "insertedId" : ObjectId("5e04e186150ee0e76c06a051") }Following is the query to display all documents from a collection with the help of find() method −> db.findOneMonthAgoData.find();This will produce the following output −{ "_id" : ObjectId("5e04e16c150ee0e76c06a04f"), "CustomerName" : "Chris", "PurchaseDate" : ISODate("2019-12-26T00:00:00Z") } { ... Read More
To aggregate nested documents in MongoDB, you can use $group. Let us first create a collection with documents −> db.aggregateDemo.insertOne( ... { ... "ProductInformation": [ ... { ... "Product1": [ ... { ... Amount: 50 ... }, ... { ... Amount: 90 ... ... Read More