Object-Oriented Programming (OOP)

Object-Oriented Programming (OOP) is a programming paradigm that revolves around the concept of "objects," which can encapsulate data and behavior. Here are some important concepts of OOP:

 Class
   - A class is a blueprint or template for creating objects.
   - It defines the attributes (data members) and methods (functions) that the objects will have.




Object
   - An object is an instance of a class. It is a tangible entity that represents a real-world entity.
   - Objects have attributes (characteristics or properties) and behaviors (actions or methods).



Constructor
   - A constructor is a special method that is invoked when an object is created.
   - It initializes the object's state and is often used to set initial values for the object's attributes.




Encapsulation
   - Encapsulation is the bundling of data (attributes) and the methods that operate on the data into a single unit (class).
   - It helps in controlling access to the internal details of an object and promotes data hiding.

Two main importance of encapsulation:
Data Protection
Information Hiding




Inheritance
   - Inheritance allows a new class (subclass/derived class) to inherit attributes and methods from an existing class (base class/parent class).
   - It promotes code reuse and establishes a relationship between classes.



Polymorphism
   - Polymorphism means "many forms." It allows objects of different classes to be treated as objects of a common base class.
   - It enables a single interface to represent different types or forms.


Abstraction
   - Abstraction is the process of simplifying complex systems by modeling classes based on the essential properties.
   - It involves hiding the unnecessary details and exposing only the relevant features of an object.







Method Overloading
   - Method overloading occurs when a class has multiple methods with the same name but different parameters (number or types).
   - It allows a class to perform similar operations with different inputs.





Method Overriding
   - Method overriding happens when a subclass provides a specific implementation for a method that is already defined in its superclass.
   - It allows a subclass to provide a specialized version of a method.


What is HTTP and what does it stand for?

HTTP stands for Hypertext Transfer Protocol. It is an application layer protocol used for transmitting hypermedia documents, such as HTML files, over the Internet. HTTP follows a client-server model where a client sends requests to the server and the server responds with the requested resources.

What are the main methods used in HTTP? Explain each of them.

The main methods used in HTTP are:
GET: Used to request data from a specified resource.
POST: Used to submit data to be processed to a specified resource.
PUT: Used to update a resource.
DELETE: Used to delete a specified resource.
PATCH: Used to apply partial modifications to a resource.
HEAD: Similar to GET, but only returns the response headers without the actual data.
OPTIONS: Used to describe the communication options for the target resource.
TRACE: Used to perform a message loop-back test along the path to the target resource.



What is the difference between HTTP and HTTPS?

HTTPS (Hypertext Transfer Protocol Secure) is the secure version of HTTP. The main difference is that HTTPS uses encryption to secure the data transmitted between the client and the server, whereas HTTP does not.


List of HTTP status codes:

Hypertext Transfer Protocol (HTTP) response status codes
Status codes are issued by a server in response to a client's request made to the server


All HTTP response status codes are separated into five classes or categories. The first digit of the status code defines the class of response, while the last two digits do not have any classifying or categorization role. There are five classes defined by the standard:
1xx (informational response) – the request was received, continuing process
2xx (successful) – the request was successfully received, understood, and accepted
3xx (redirection) – further action needs to be taken in order to complete the request
4xx (client error) – the request contains bad syntax or cannot be fulfilled
5xx (server error) – the server failed to fulfill an apparently valid request

Some important status codes:

200 OK
Standard response for successful HTTP requests. The actual response will depend on the request method used. Example GET , POST request etc.

201 Created
The request has been fulfilled, resulting in the creation of a new resource.

202 Accepted
The request has been accepted for processing, but the processing has not been completed. The request might or might not be eventually acted upon, and may be disallowed when processing occurs.

400 Bad Request
The server cannot or will not process the request due to an apparent client error (e.g., malformed request syntax, size too large, invalid request message framing, or deceptive request routing).

401 Unauthorized
Similar to 403 Forbidden, but specifically for use when authentication is required and has failed or has not yet been provided.


403 Forbidden
The request contained valid data and was understood by the server, but the server is refusing action. This may be due to the user not having the necessary permissions for a resource or needing an account of some sort, or attempting a prohibited action (e.g. creating a duplicate record where only one is allowed).

404 Not Found
The requested resource could not be found but may be available in the future. Subsequent requests by the client are permissible.

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Predict the Output:

What will be the output of typeof [] and typeof {} in JavaScript?
Explain the difference between == and === operators in JavaScript with examples.

Predict the output of
console.log(0 == false);
console.log(0 === false);
console.log("" == false);

What happens when you execute
console.log(3 + 4 + "5") and
console.log("5" + 3 + 4)

Explain the concept of hoisting in JavaScript and predict the output of

console.log(x)
var x = 5;

What will be the output of
console.log(typeof NaN) and
console.log(NaN === NaN)

Predict the output of following code:

console.log([2] === [2]);

Predict the output of
console.log(2 + "2");
console.log(2 - "2");
console.log(2 * "2");


Predict the output of the following code:

for (var i = 0; i < 5; i++) {
   setTimeout(function() {
   console.log(i);
   }, 1000);
}




What will be the output of the following code:

const person = {
 name: "John",
 age: 30,
 address: {
 city: "New York",
 zip: 10001
    }
    };

const newPerson = { ...person };
newPerson.name = "Jane";
newPerson.address.city = "Los Angeles";

console.log(person.name);
console.log(person.address.city);



Explain the event loop in JavaScript and predict the order of execution for the following code:


console.log("Start");

setTimeout(function() {
 console.log("Timeout 1");
}, 0);

Promise.resolve().then(function() {
 console.log("Promise 1");
});

console.log("End");








Database Modeling:

Database modeling is the process of creating a structured representation of a database, defining its structure, relationships, constraints, and other characteristics. It involves designing the schema of the database to ensure efficient storage, retrieval, and manipulation of data.








Conceptual Data Model:

    The conceptual data model describes the database at a very high level and is useful to understand the needs or requirements of the database. It is this model that is used in the requirement-gathering process i.e. before the Database Designers start making a particular database. One such popular model is the Entity-Relationship model (ER model). The ER model specializes in entities, relationships, and even attributes that are used by database designers.


Entity-Relationship Model( ER Model): It is a high-level data model which is used to define the data and the relationships between them. It is basically a conceptual design of any database which is easy to design the view of data.
Components of ER Model:

Entity: An entity is referred to as a real-world object. It can be a name, place, object, class, etc. These are represented by a rectangle in an ER Diagram.
Attributes: An attribute can be defined as the description of the entity. These are represented by Ellipse in an ER Diagram. It can be Age, Roll Number, or Marks for a Student.
Relationship: Relationships are used to define relations among different entities. Diamonds and Rhombus are used to show Relationships.
Consider a basic scenario of a library management system. Here's a simple ER diagram for this scenario:

In this ER diagram:

There are three entities: Book, Author, and Library.
Each Book has attributes such as BookID, Title, ISBN, and Year.
Each Author has attributes such as AuthorID, Name, and Country.
Each Library has attributes such as LibraryID, Name, and Location.
There are two relationships:
The Book entity is related to the Author entity by a many-to-many relationship, as one book can have multiple authors, and one author can write multiple books. This relationship is represented by the junction entity BookAuthor with attributes BookID and AuthorID.
The Book entity is related to the Library entity by a one-to-many relationship, as one library can have multiple books, but each book belongs to only one library. This relationship is represented by the LibraryID attribute in the Book entity.


Representational Data Model:
This type of data model is used to represent only the logical part of the database and does not represent the physical structure of the database.
The representational data model allows us to focus primarily on the design part of the database. A popular representational model is a Relational model.
In the Relational Model, we basically use tables to represent our data and the relationships between them. It is a theoretical concept whose practical implementation is done in the Physical Data Model.


Physical Data Model:
 The physical Data Model is used to practically implement Relational Data Model.
Ultimately, all data in a database is stored physically on a secondary storage device such as discs and tapes.
This is stored in the form of files, records, and certain other data structures.
It has all the information on the format in which the files are present and the structure of the databases, the presence of external data structures, and their relation to each other.







Database Queries:

SQL

1) Retrieve the names of employees with a salary greater than $50,000:

SELECT name FROM employees WHERE salary > 50000;

2) Update the salary of an employee with ID 101 to $60,000:

UPDATE employees SET salary = 60000 WHERE employee_id = 101;

3) Count the number of employees in each department:

SELECT department, COUNT(*) FROM employees GROUP BY department;

4) Delete all records from the "customers" table where the country is 'USA':

DELETE FROM customers WHERE country = 'USA';

5) To display the remaining data in the user entity table, excluding the first five rows which contain corrupt data.

SELECT * FROM user  OFFSET 5;

6) Finding the average age of users:

SELECT AVG(age) AS average_age FROM user;

7) Finding users who joined in the last month:

SELECT *
FROM user
WHERE join_date >= DATE_SUB(CURRENT_DATE, INTERVAL 1 MONTH);

8) Finding users with specific email domains:

SELECT *
FROM user
WHERE email LIKE '%@example.com';

9) Delete all records from the "customers" table where the country is 'USA':

DELETE FROM customers WHERE country = 'USA';

10) Retrieve all columns from the "employees" table:

SELECT * FROM employees;