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To prevent duplicates of multiple fields, use ensureIndex() and set unique:true. Let us create a collection with documents −> db.demo272.ensureIndex({"FirstName":1, "Subject":1}, {unique:true}); { "createdCollectionAutomatically" : true, "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 } > db.demo272.insertOne({"FirstName":"Chris", "Subject":"MySQL"}); { "acknowledged" : true, "insertedId" : ObjectId("5e48232a1627c0c63e7dbabf") } > db.demo272.insertOne({"FirstName":"Chris", "Subject":"MongoDB"}); { "acknowledged" : true, "insertedId" : ObjectId("5e48232f1627c0c63e7dbac0") } > db.demo272.insertOne({"FirstName":"David", "Subject":"MySQL"}); { "acknowledged" : true, "insertedId" : ObjectId("5e48233a1627c0c63e7dbac1") } > db.demo272.insertOne({"FirstName":"Chris", "Subject":"MySQL"}); 2020-02-15T22:28:55.137+0530 E QUERY [js] WriteError: E11000 duplicate key error collection: test.demo272 index: FirstName_1_Subject_1 dup key: { : ... Read More
Set a custom variable and use update() along with $inc to increment. Let us create a collection with documents −> db.demo271.insertOne({"Marks":56}); { "acknowledged" : true, "insertedId" : ObjectId("5e4821211627c0c63e7dbabc") } > db.demo271.insertOne({"Marks":78}); { "acknowledged" : true, "insertedId" : ObjectId("5e4821241627c0c63e7dbabd") } > db.demo271.insertOne({"Marks":72}); { "acknowledged" : true, "insertedId" : ObjectId("5e48212b1627c0c63e7dbabe") }Display all documents from a collection with the help of find() method −> db.demo271.find();This will produce the following output −{ "_id" : ObjectId("5e4821211627c0c63e7dbabc"), "Marks" : 56 } { "_id" : ObjectId("5e4821241627c0c63e7dbabd"), "Marks" : 78 } { "_id" : ObjectId("5e48212b1627c0c63e7dbabe"), "Marks" : 72 }Following is the query ... Read More
Let us first create a collection with documents −> db.demo270.insertOne({"ClientName":"Chirs", "Age":34}); { "acknowledged" : true, "insertedId" : ObjectId("5e481e371627c0c63e7dbab8") } > db.demo270.insertOne({"ClientName":"David", "Age":31}); { "acknowledged" : true, "insertedId" : ObjectId("5e481e3d1627c0c63e7dbab9") } > db.demo270.insertOne({"ClientName":"Bob", "Age":31}); { "acknowledged" : true, "insertedId" : ObjectId("5e481e431627c0c63e7dbaba") } > db.demo270.insertOne({"ClientName":"Carol", "Age":36}); { "acknowledged" : true, "insertedId" : ObjectId("5e481e491627c0c63e7dbabb") }Display all documents from a collection with the help of find() method −> db.demo270.find();This will produce the following output −{ "_id" : ObjectId("5e481e371627c0c63e7dbab8"), "ClientName" : "Chirs", "Age" : 34 } { "_id" : ObjectId("5e481e3d1627c0c63e7dbab9"), "ClientName" : "David", "Age" : 31 } ... Read More
The JShell tool also called REPL(Read-Evaluate-Print-Loop) introduced in Java 9 that allows us to execute Java code and getting immediate results. We can quickly evaluate expressions or short algorithms without creating a new project, compile or build it. With the help of JShell, we can execute expressions, use imports, define classes, methods, and variables.We can list out all the classes, interfaces and enums defined in the current JShell session by using "/types" command.In the below code snippet, we have created the "Test" class, "TestInterface" interface, and enum "EnumTest" in the JShell tool.C:\Users\User> jshell | Welcome to JShell -- Version 9.0.4 | For ... Read More
For this, simply use find(). Set the fields you don’t want to select to 0. Let us create a collection with documents −> db.demo269.insertOne({StudentId:101, StudentSubject:"MySQL"}); { "acknowledged" : true, "insertedId" : ObjectId("5e481caa1627c0c63e7dbab4") } > db.demo269.insertOne({StudentId:102, StudentSubject:"Java"}); { "acknowledged" : true, "insertedId" : ObjectId("5e481cb11627c0c63e7dbab5") } > db.demo269.insertOne({StudentId:103, StudentSubject:"MongoDB"}); { "acknowledged" : true, "insertedId" : ObjectId("5e481cb21627c0c63e7dbab6") } > db.demo269.insertOne({StudentId:104, StudentSubject:"C"}); { "acknowledged" : true, "insertedId" : ObjectId("5e481cb21627c0c63e7dbab7") }Display all documents from a collection with the help of find() method −> db.demo269.find();This will produce the following output −{ "_id" : ObjectId("5e481caa1627c0c63e7dbab4"), "StudentId" : 101, "StudentSubject" ... Read More
To search a value, simply use $where in MongoDB. Let us create a collection with documents −> db.demo268.insertOne( ... { ... "details" : { ... "101" : "John", ... "1001" : "Bob" ... } ... } ...); { "acknowledged" : true, "insertedId" : ObjectId("5e4816141627c0c63e7dbaaf") }Display all documents from a collection with the help of find() method −> db.demo268.find();This will produce the following output −{ "_id" : ObjectId("5e4816141627c0c63e7dbaaf"), "details" : { "101" : "John", "1001" : "Bob" } }Following is the query to search a ... Read More
For descending order, use -1, which specifies sorting order for sort(), Let us create a collection with documents −> db.demo267.insertOne({id:100, "Name":"Chris"}); { "acknowledged" : true, "insertedId" : ObjectId("5e4811951627c0c63e7dbaab") } > db.demo267.insertOne({id:100, "Name":"Adam"}); { "acknowledged" : true, "insertedId" : ObjectId("5e48119e1627c0c63e7dbaac") } > db.demo267.insertOne({id:100, "Name":"David"}); { "acknowledged" : true, "insertedId" : ObjectId("5e4811a51627c0c63e7dbaad") } > db.demo267.insertOne({id:100, "Name":"Bob"}); { "acknowledged" : true, "insertedId" : ObjectId("5e4811ab1627c0c63e7dbaae") }Display all documents from a collection with the help of find() method −> db.demo267.find().pretty(); { "_id" : ObjectId("5e4811951627c0c63e7dbaab"), "id" : 100, "Name" : "Chris" } { "_id" ... Read More
The $not operator won’t invert a complex expression. Therefore, use $and or $or with $ne operator.Let us create a collection with documents −> db.demo266.insertOne({"active1":true, "active2":false}); { "acknowledged" : true, "insertedId" : ObjectId("5e480f4b1627c0c63e7dbaa7") } > db.demo266.insertOne({"active1":true, "active2":true}); { "acknowledged" : true, "insertedId" : ObjectId("5e480f501627c0c63e7dbaa8") } > db.demo266.insertOne({"active1":false, "active2":false}); { "acknowledged" : true, "insertedId" : ObjectId("5e480f561627c0c63e7dbaa9") } > db.demo266.insertOne({"active1":false, "active2":true}); { "acknowledged" : true, "insertedId" : ObjectId("5e480f701627c0c63e7dbaaa") }Display all documents from a collection with the help of find() method −> db.demo266.find();This will produce the following output −{ "_id" : ObjectId("5e480f4b1627c0c63e7dbaa7"), "active1" : true, "active2" ... Read More
Suppose we have a non-negative integer num that is represented as a string, we have to remove k digits from the number so that the new number is the smallest possible. So if the input is like “1432219” and k = 3, then the result will be “1219”.To solve this, we will follow these steps −Define a stack st, create an empty string retn := size of numfor i in range 0 to n – 1while k is non zero and stack is not empty and top of stack > num[i]delete from the stack and decrease k by 1insert num[i] ... Read More
To update only certain fields, use $set. Let us create a collection with documents −> db.demo265.insertOne({"id":101, "Name":"Chris"}); { "acknowledged" : true, "insertedId" : ObjectId("5e480d781627c0c63e7dbaa4") } > db.demo265.insertOne({"id":102, "Name":"Bob"}); { "acknowledged" : true, "insertedId" : ObjectId("5e480d7d1627c0c63e7dbaa5") } > db.demo265.insertOne({"id":103, "Name":"David"}); { "acknowledged" : true, "insertedId" : ObjectId("5e480d841627c0c63e7dbaa6") }Display all documents from a collection with the help of find() method −> db.demo265.find();This will produce the following output −{ "_id" : ObjectId("5e480d781627c0c63e7dbaa4"), "id" : 101, "Name" : "Chris" } { "_id" : ObjectId("5e480d7d1627c0c63e7dbaa5"), "id" : 102, "Name" : "Bob" } { "_id" : ObjectId("5e480d841627c0c63e7dbaa6"), "id" : 103, ... Read More