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‎MongoDB/Interview/3.Query_Operations.md

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# MongoDB Questions (Query Operations and Projection)
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## Q1. What are MongoDB query operators? Provide some examples.
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MongoDB query operators are used to specify criteria in queries and to perform a variety of operations, such as comparison, logical operations, and element evaluation.
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* **$eq**: Matches values that are equal to a specified value.
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* **$gt**: Matches values that are greater than a specified value.
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* **$lt**: Matches values that are less than a specified value.
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* **$in**: Matches any of the values specified in an array.
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* **$and**: Joins query clauses with a logical AND.
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* **$or**: Joins query clauses with a logical OR.
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## Q2. How does the $regex operator work in MongoDB? Provide an example query.
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The `$regex` operator provides regular expression capabilities for pattern matching strings in queries.
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```json
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{ "name": { "$regex": "^A", "$options": "i" } }
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```
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## Q3. Explain how to use the $regex operator to find documents where the 'name' field starts with 'apple'.
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```json
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db.products.find({ name: /^apple/ });
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```
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## Q3. What is projection in MongoDB and why is it useful?
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Projection in MongoDB is the process of selecting specific fields to return in query results. It is useful for:
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* Reducing the amount of data transferred over the network.
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* Improving query performance by retrieving only necessary data.
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```json
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db.collection.find({ "status": "active" }, { "name": 1, "email": 1, "_id": 0 })
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```
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## Q4. How do you perform a projection to exclude specific fields in MongoDB?
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To exclude specific fields in a projection, set their values to 0.
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```json
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db.collection.find({}, { "password": 0, "creditCardNumber": 0 })
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```
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>This query returns all fields except password and creditCardNumber.
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## Q5. What is the purpose of the $slice operator in MongoDB projection? Provide an example.
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The `$slice` operator limits the number of elements in an array returned by a query.
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```json
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db.collection.find({}, { "comments": { "$slice": 5 } })
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```
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## Q6. Describe how the $elemMatch projection operator works. Provide an example.
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The $elemMatch projection operator limits the contents of an array field to contain only the first element that matches the specified query condition.
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```json
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db.collection.find(
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{ "results": { "$elemMatch": { "score": { "$gt": 80 } } } },
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{ "results": { "$elemMatch": { "score": { "$gt": 80 } } } }
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)
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```
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This query matches documents where the `results` array contains at least one element with a `score` greater than 80, and projects only that element.
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## Q7. What is the purpose of the $ operator in projections, and how does it differ from its use in updates?
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In projections, the `$` operator limits the output to contain only the first array element that matches the query condition. In updates, it identifies the first matching element in an array for modification.
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```json
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db.collection.find(
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{ "results.score": { "$gt": 80 } },
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{ "results.$": 1 }
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)
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```
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This query returns documents where results.score is greater than 80, projecting only the first matching results array element.
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```json
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db.collection.updateOne(
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{ "results.score": { "$gt": 80 } },
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{ "$set": { "results.$.score": 90 } }
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)
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```
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This query updates the `score` to 90 for the first element in the `results` array that matches the condition `score > 80`.
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<!---
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Adarsh
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2nd August 2024
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7:10 AM
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(09:24)
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--->

‎ReactJS/Interview/100_React_Question/6.Routing.md

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# 100 React Questions (Routing)
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## Q1. What is Routing and Router in React? `V. IMP.`
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### Routing
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Routing allows you to create a single-page web application with navigation, without the need for a full-page refresh.
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### React Router
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React Router is a library for handling routing and enables navigation and rendering of different components based on the URL.
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## Q2. How to Implement Routing in React? `V. IMP.`
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* Install React-Router
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* Create Navigation
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* Create Routes
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```bash
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npm install react-router-dom
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```
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### Navbar.js
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```javascript
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import React from 'react';
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import { Routes, Route, Link } from 'react-router-dom';
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// Elements or imported components
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const Home = () => <h2>Home</h2>;
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const About = () => <h2>About</h2>;
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const Contact = () => <h2>Contact</h2>;
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const AppRoute = () => (
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<div>
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{/* Navigation links */}
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<nav>
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<ul>
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<li><Link to="/">Home</Link></li>
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<li><Link to="/about">About</Link></li>
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<li><Link to="/contact">Contact</Link></li>
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</ul>
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</nav>
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```
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### index.js
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```javascript
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import AppRoute from './Others/AppRoute';
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import { BrowserRouter as Router } from 'react-router-dom';
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const root = ReactDOM.createRoot(document.getElementById('root'));
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root.render(
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<Router>
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<AppRoute />
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</Router>
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);
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{/* Routes */}
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<Routes>
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<Route path="/" element={<Home />} />
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<Route path="/about" element={<About />} />
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<Route path="/contact" element={<Contact />} />
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</Routes>
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</div>
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);
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export default AppRoute;
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```
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## Q3. What are the roles of <Routes> & <Route> component in React Routing? `V. IMP.`
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* The <Routes> component is used as the root container for declaring your `collection of routes`.
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* The <Route> component is used to define a route and specify the component that should render when the `route matches`.
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* For example, in this code if user enter "websitename.com/about [invalid URL removed]" in url, then matching "About" component will be rendered.
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```javascript
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import React from 'react';
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import { Routes, Route, Link } from 'react-router-dom';
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<Routes>
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<Route path="/" element={<Home />} />
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<Route path="/about" element={<About />} />
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<Route path="/contact" element={<Contact />} />
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</Routes>
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```
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## Q4. What are Route Parameters in React Routing?
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Route parameters in React Router are a way to pass dynamic values(data) to the component `as part of the URL path`.
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```javascript
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{/* userId is the route parameter */}
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<Route path="/users/:userId" component={UserProfile}/>
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```
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## Q5. What is the role of Switch Component in React Routing?
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* Switch component ensures that only the `first matching <Route> is rendered` and rest are ignored.
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* For example, Switch is commonly used to handle 404 or "not fount" routes.
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```javascript
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import {Switch, Route} from 'react-router-dom';
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<Switch>
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<Route path="/users" component={UserList}/>
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<Route path="/users/:id" component={UserProfile}/>
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</Switch>
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```
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## Q6. What is the role of exact prop in React Routing?
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exact prop is used with the <Route> component to `match exactly` to the provided path.
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```javascript
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{/*Without exact (default behavior)*/}
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{/*Match /about, /about/team, /about/contact, etc.*/}
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<Route path="/about" component={About} />
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```
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```javascript
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{/*With exact*/}
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{/*Only match /about*/}
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<Route path="/about" exact component={About} />
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```
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<!---
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Adarsh
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2nd August 2024
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09:52 PM
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(20:29)
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--->

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