DEV Community: Tharunya K R

How a request originates from client and reaches the server

Tharunya K R — Wed, 25 Mar 2026 18:36:55 +0000

1.You type a website

Example: google.com

2.Browser finds IP

It asks DNS:
“Where is this website?”

Gets IP address

3.Request is sent

Your browser sends request to server
Like: “Give me the webpage”

4.Server receives request

Server understands what you want

5.Server processes

Fetch data from database
Perform calculations
Check authentication

6.Server sends response

Along with HTML , CSS ,JavaScript

7.Browser shows output

You see the webpage on screen

IP address and Subnet

Tharunya K R — Wed, 25 Mar 2026 18:22:50 +0000

IP Address

It is a unique number given to each device connected to a network
Example :192.168.1.1

Two Types Of IP Address

IPV4
IPV6

IPV4
It is commonly used
32-bit address
Example: 192.168.1.1

IPV6
New version
128-bit address

Subnet

It is a smaller division of a large network

Example

192.168.1.0
We can divide into smaller subnets like:
192.168.1.1 → 192.168.1.254

Durability

Tharunya K R — Wed, 25 Mar 2026 17:56:30 +0000

Durability means once transaction is commit the data is permanently saved
Example
Transfer 100 from A to B
A=1000
B=500
1.after transaction A=900
B=600

2.System Crashes Happen

3.After Restart
A=900
B=600

Isolation

Tharunya K R — Wed, 25 Mar 2026 17:41:16 +0000

Isolation means when multiple transaction run at the same time they should not affect each other

Example
Initial Data
A=1000

Without Isolation

T1 = Deduct 800

T2 = Deduct 500

1000-800-500 = -300

With Isolation
T1 = Deduct 800
remaining 200
T2 = deduct 500
200 - 500 (Not allowed)

Consistency

Tharunya K R — Wed, 25 Mar 2026 17:11:46 +0000

Consistency means after transaction database must give the correct and valid answer
Example :
if A = Rs 100 , B = Rs 200
Total = Rs 300
After transfer Total = Rs 300
if it is the correct value then it is inconsistency

Create table example

Table Used
CREATE TABLE accounts (
id SERIAL PRIMARY KEY,
name TEXT NOT NULL,
balance INT NOT NULL CHECK (balance >= 0)
);
balance >= 0

Sample Data

INSERT INTO accounts (name, balance)
VALUES
('Alice', 1000),
('Bob', 500);

Valid Transaction

UPDATE accounts SET balance = balance - 100 WHERE name = 'Alice';

Result

Alice = 900
It is valid

Invalid Transaction

UPDATE accounts SET balance = -500 WHERE name = 'Alice';

Eror Occur Because negative value is occuring

Atomicity

Tharunya K R — Wed, 25 Mar 2026 16:46:46 +0000

Atomicity means it either happen completely or it does not happen anything
If there is no error it give the correct result
If there is an error it rollback(cancel) return to the initial state
Testing error will happen to check the atomicity

Example :
Create an table

CREATE TABLE accounts (
id SERIAL PRIMARY KEY,
name TEXT NOT NULL,
balance INT NOT NULL CHECK (balance >= 0)
);
Sample Data
INSERT INTO accounts (name, balance)
VALUES
('Alice', 1000),
('Bob', 500);

Correct Transaction :

BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE name = 'Alice';
UPDATE accounts SET balance = balance + 100 WHERE name = 'Bob';
COMMIT;

Result :
Alice : 900
Bob : 600

Testing Atomicity :

BEGIN; UPDATE accounts SET balance = balance - 100 WHERE name = 'Alice';
UPDATE accounts SET balancee = balance + 100 WHERE name = 'Bob';
COMMIT;

Here error happens ibstead of balance it shows balancee
So Rollback happens it cancel every transaction and it return the initial state.

DB Problems

Tharunya K R — Wed, 25 Mar 2026 14:28:45 +0000

1.Find all movies where the special features are not listed (i.e., special_features is NULL).

SELECT title FROM film WHERE special features = NULL;
Meaning : It returns the title of the movie where the special feature is null

2.Find all movies where the rental duration is more than 7 days.

SELECT title , rental FROM film WHERE rental_duration > 7;
Meaning : It returns the title of the movie and rental_duration more than 7 days

3.Find all movies that have a rental rate of $4.99 and a replacement cost of more than $20

SELECT title , rental_rate , replacement_cost FROM film WHERE rental_rate = 4.99 AND replacement_cost > 20;
Meaning : It returns the title of the movie , rental_rate is equal to 4.99 and replacement_cost more than 20

4.Find all movies that have a rental rate of $0.99 or a rating of 'PG-13'.

SELECT title , rental_rate , rating FROM film WHERE rental_rate = 0.99 OR rating = 'PG-13';
Meaning : It returns title , rental_rate with 0.99 and rating with PG-13

5.Retrieve the first 5 rows of movies sorted alphabetically by title.

SELECT title FROM film ORDER BY title ASC LIMIT 5;
Meaning : It Return the first 5 rows from the title column

6.Skip the first 10 rows and fetch the next 3 movies with the highest replacement cost.

SELECT title , replacement_cost FROM film ORDER BY replacement_cost DESC OFFSET 10 LIMIT 3;
Meaning : It returns the title , replacement_cost with highest 3 and skip the first 10 rows

7.Find all movies where the rating is either 'G', 'PG', or 'PG-13'.

SELECT title , rating FROM film WHERE rating = 'G' OR 'PG' OR 'PG-13';
Meaning : It returns all the title with ratings of G or PG or PG-13

8.Find all movies with a rental rate between $2 and $4.

SELECT title , rental_rate FROM film WHERE rental_rate > 2 AND rental_rate < 4;
Meaning : It return the title with rental_rate between 2 and 4

9.Find all movies with titles that start with 'The'.

SELECT title FROM film WHERE title LIKE 'The%';
Meaning : It return all the title where title name start with "The"

10.Find the first 10 movies with a rental rate of $2.99 or $4.99, a rating of 'R', and a title containing the word "Love".

SELECT title , rental_rate , rating FROM film WHERE rental_rate = 2.99 OR rental_rate = 4.99 AND renting = 'R' AND title LIKE '%Love%' LIMIT 10;
Meaning : It return the title with LOVE word and rental rate with 2.99 or 4.99 and renting is equl to "R"

11.Find all movies where the title contains the % symbol.

SELECT title FROM film WHERE title LIKE '%\%%' ESCAPE '\';
Meaning: It returns all movie titles that contain the percent (%) symbol anywhere in the title

12.Find all movies where the title contains an underscore (_).

SELECT title FROM film WHERE title LIKE '%_%' ESCAPE '\';
Meaning: It returns all movie titles that contain an underscore (_) anywhere

13.Find all movies where the title starts with "A" or "B" and ends with "s".

SELECT title FROM film WHERE (title LIKE 'A%' OR title LIKE 'B%') AND title LIKE '%s';
Meaning: It returns all movie titles starting with "A" or "B" and ending with "s"

14.Find all movies where the title contains "Man", "Men", or "Woman".

SELECT title FROM film WHERE title LIKE '%Man%' OR title LIKE '%Men%' OR title LIKE '%Woman%';
Meaning: It returns all movie titles that contain the words "Man", "Men", or "Woman" anywhere in the title

15.Find all movies with titles that contain digits (e.g., "007", "2", "300").

SELECT title FROM film WHERE title ~ '[0-9]';
Meaning: It returns all movie titles that contain any number (0–9) in the title

16.Find all movies with titles containing a backslash ()

SELECT title FROM film WHERE title LIKE '%\%' ESCAPE '\';
Meaning: It returns all movie titles that contain a backslash () anywhere

17.Find all movies where the title does contain the words "Love" or "Hate".

SELECT title FROM film WHERE title LIKE '%Love%' OR title LIKE '%Hate%';
Meaning: It returns all movie titles that contain "Love" or "Hate" anywhere in the title.

18.Find the first 5 movies with titles that end with "er", "or", or "ar".

SELECT title FROM film WHERE title LIKE '%er' OR title LIKE '%or' OR title LIKE '%ar' LIMIT 5;
Meaning: It returns the first 5 movie titles that end with "er", "or", or "ar"

Database Problems

Tharunya K R — Tue, 24 Mar 2026 17:15:11 +0000

Query all columns for a city in CITY with the ID 1661.

SELECT * FROM CITY WHERE ID=1661;
Meaning : Return all columns from CITY table where ID is equal to 1661

Query all columns for all American cities in the CITY table with populations larger than 100000. The CountryCode for America is USA.

SELECT * FROM CITY WHERE CountryCode = 'USA' AND Population >100000 ;
Meaning : Return all columns from CITY table where CountryCode is USA and population is greater than 100000

Query all attributes of every Japanese city in the City table. The COUNTRYCODE for Japan is JPN.

SELECT * FROM CITY WHERE CountryCode='JPN';
Meaning : Return all columns from CITY table where CountryCode is JPN

Find the difference between the total number of CITY entries in the table and the number of distinct CITY entries in the table.

SELECT COUNT(CITY) - COUNT(DISTINCT CITY) AS difference FROM STATION;
Meaning : Return the difference between total number of city entries and unique city entries from the STATION table

Query the NAME field for all American cities in the CITY table with populations larger than 120000. The CountryCode for America is USA.

SELECT NAME FROM CITY WHERE COUNTRYCODE = 'USA' AND POPULATION > 120000;
Meaning : Return only the NAME column of cities from CITY table where CountryCode is USA and population is greater than 120000

Query the names of all the Japanese cities in the CITY table. The COUNTRYCODE for Japan is JPN.

SELECT NAME FROM CITY WHERE COUNTRYCODE = 'JPN';
Meaning : Return only the NAME column of cities from CITY table where CountryCode is JPN

Query the list of CITY names from STATION that do not start with vowels. Your result cannot contain duplicates.

SELECT DISTINCT CITY FROM STATION WHERE CITY NOT LIKE 'A%' AND CITY NOT LIKE 'E%' AND CITY NOT LIKE 'I%' AND CITY NOT LIKE 'O%' AND CITY NOT LIKE 'U%';
Meaning : Return unique city names from STATION table where the city name not start with a vowel

Query all columns (attributes) for every row in the CITY table.

SELECT * FROM CITY;
Meaning : Return all columns and all rows from the CITY table

Query a list of CITY and STATE from the STATION table.

SELECT CITY , STATE FROM STATION;
Meaning : Return only the CITY and STATE columns from the STATION table

Database Task

Tharunya K R — Tue, 24 Mar 2026 16:45:01 +0000

Retrieve film titles and their rental rates. Use column aliases to rename title as "Movie Title" and rental_rate as "Rate".

SELECT title AS "Movie Title" , rental_rate AS "Rate" FROM film;

Meaning : Changing the name of the column title as Movie Title and rental_rate as Rate From film table

List customer names and their email addresses. Alias first_name and last_name as "First Name" and "Last Name".

SELECT first_name AS "First Name", last_name AS "Last Name" FROM customer;
SELECT first_name , last_name , email FROM customer;

Meaning : Return only first name , last name and email from the customer table

Get a list of films sorted by rental rate in descending order. If two films have the same rental rate, sort them alphabetically by title.

SELECT title, rental_rate FROM film ORDER BY rental_rate DESC, title ASC;

Meaning : Return title and rental_rate from film table and rental_rate by descending , title by sorted

Retrieve actor names sorted by last name, then first name.

SELECT first_name, last_name FROM actor ORDER BY last_name ASC, first_name ASC;

Meaning : Return last_name and first_name from film table by ascending order

List all unique replacement costs from the film table.

SELECT DISTINCT replacement_cost FROM film;

Meaning : Return all non-duplicate values from film table

List all films' title and length in minutes. Alias length as "Duration
(min)".

SELECT title, length AS "Duration (min)" FROM film;

Meaning : Return title and change length name as Duration(min) from film table

Retrieve customer first and last names along with their active status. Alias active as "Is Active".

SELECT first_name, last_name, active AS "Is Active"
FROM customer;

Meaning : Return first_name , last_name and active status change as Active from customer table

Retrieve the list of film categories sorted alphabetically.

SELECT name AS "Category" FROM category ORDER BY name ASC;

Meaning : Return name change as category and arrange in ascending order

List films by length, sorted in descending order. Include only the title and length.

SELECT title, length FROM film ORDER BY length DESC;

Meaning : Return title and length from film table and sort length in descending order (longest films first)

Retrieve all actor names, sorted by their first name in descending order.

SELECT first_name, last_name FROM actor ORDER BY first_name DESC;

Meaning : Return first_name and last_name from actor table and sort by first_name in descending order

List all unique ratings available in the film table.

SELECT DISTINCT rating FROM film;

Meaning : Return all unique (non-duplicate) ratings from film table

Find all unique rental durations from the film table.

SELECT DISTINCT rental_duration FROM film;

Meaning : Return all unique rental_duration values from film table

Retrieve the first unique customer ID based on active status. Include the customer_id and active columns, and order by customer_id.

SELECT customer_id, active FROM customer ORDER BY customer_id LIMIT 1;

Meaning : Return first customer_id and active status from customer table based on ascending order

List the earliest rental date for each customer. Include customer_id and rental_date, and order by customer_id.

SELECT customer_id, MIN(rental_date) AS rental_date FROM rental GROUP BY customer_id ORDER BY customer_id;

*Meaning : Return customer_id and earliest rental_date for each customer using MIN function
*
List the 10 shortest films by length. Include the title and length.

SELECT title, length FROM film ORDER BY length ASC LIMIT 10;
**
**Meaning : Return 10 films with shortest length from film table

Get the top 5 customers with the highest customer_id. Include the first and last name.

SELECT first_name, last_name FROM customer ORDER BY customer_id DESC LIMIT 5;

Meaning : Return top 5 customers with highest customer_id sorted in descending order

Retrieve all unique values of store_id from the inventory table.

SELECT DISTINCT store_id FROM inventory;

Meaning : Return all unique store_id values from inventory table

Find all unique replacement_cost values in the film table. Sort the results in ascending order.

SELECT DISTINCT replacement_cost FROM film ORDER BY replacement_cost ASC;

*Meaning : Return unique replacement_cost values sorted in ascending order
*
List the first rental date for each store. Include store_id and rental_date, and sort by store_id.

SELECT store_id, MIN(rental_date) AS rental_date FROM rental GROUP BY store_id ORDER BY store_id;

Meaning : Return earliest rental_date for each store using MIN function

Retrieve a list of film ratings sorted alphabetically and include only unique values.

SELECT DISTINCT rating FROM film ORDER BY rating ASC;

Meaning : Return unique film ratings sorted alphabetically

List films by rating in ascending order and length in descending order.

SELECT title, rating, length FROM film ORDER BY rating ASC, length DESC;

*Meaning : Return films sorted by rating (ascending) and length (descending)
*
Retrieve actor names sorted by last_name in ascending order and first_name in descending order.

SELECT first_name, last_name FROM actor ORDER BY last_name ASC, first_name DESC;
**
**Meaning : Return actor names sorted by last_name ascending and first_name descending

List films ordered by replacement_cost in ascending order and rental_rate in descending order.

SELECT title, replacement_cost, rental_rate FROM film ORDER BY replacement_cost ASC, rental_rate DESC;

Meaning : Return films sorted by replacement_cost ascending and rental_rate descending

Retrieve customer names sorted by last_name ascending and first_name descending.

SELECT first_name, last_name FROM customer ORDER BY last_name ASC, first_name DESC;

Meaning : Return customer names sorted by last_name ascending and first_name descending

List all rentals sorted by customer_id ascending and rental_date descending.

SELECT customer_id, rental_date FROM rental ORDER BY customer_id ASC, rental_date DESC;

Meaning : Return rentals sorted by customer_id ascending and rental_date descending

Retrieve a list of films ordered by rental_duration ascending and title descending.

SELECT title, rental_duration FROM film ORDER BY rental_duration ASC, title DESC;

Meaning : Return films sorted by rental_duration ascending and title descending

Removing Duplicates from a Sorted Linked List

Tharunya K R — Sun, 22 Mar 2026 14:33:25 +0000

Problem

Given a sorted singly linked list, remove duplicate nodes so that each element appears only once.

Do not use extra space.
The linked list is already sorted, so duplicates will be adjacent.
Example 1
Input: 2 -> 2 -> 4 -> 5
Output: 2 -> 4 -> 5

Explanation: The value 2 appears twice, so one duplicate is removed.

Example 2
Input: 2 -> 2 -> 2 -> 2 -> 2
Output: 2

Explanation: All nodes have value 2, so only one node remains.

Approach

Start from the head node.
Traverse the linked list.
If the current node value is equal to the next node value, skip the next node.
Otherwise move to the next node.
Continue until the end of the list.

Python Code

def removeDuplicates(head):
curr = head

while curr and curr.next:
    if curr.data == curr.next.data:
        curr.next = curr.next.next  # remove duplicate
    else:
        curr = curr.next           # move forward

return head

Output
2 -> 4 -> 5

Finding the Largest Sum of a Subarray

Tharunya K R — Sun, 22 Mar 2026 14:25:21 +0000

Problem

Given an integer array arr[], find the maximum sum of a subarray (containing at least one element).
A subarray is a continuous part of the array.

Example 1
Input: arr = [2, 3, -8, 7, -1, 2, 3]
Output: 11
Explanation: Subarray [7, -1, 2, 3] gives the maximum sum 11.

Example 2
Input: arr = [-2, -4]
Output: -2
Explanation: Subarray [-2] has the maximum sum.

Approach (Kadane’s Algorithm)

Start with the first element as the current and maximum sum.
Traverse the array from the second element.
At each step:
Either start a new subarray from the current element.
Or extend the existing subarray.
Update the maximum sum if a larger sum is found.

Python Code

class Solution:
def maxSubarraySum(self, arr):
max_sum = arr[0]
curr_sum = 0

    for num in arr:
        curr_sum += num

        if curr_sum > max_sum:
            max_sum = curr_sum

        if curr_sum < 0:
            curr_sum = 0

    return max_sum

Output
11
-2
25

Maximum Subarray Sum

Tharunya K R — Sun, 22 Mar 2026 14:21:29 +0000

Problem

Given an integer array arr[], find the maximum sum of a contiguous subarray.

The subarray must contain at least one element.
Example 1
Input: arr = [2, 3, -8, 7, -1, 2, 3]
Output: 11

Explanation: Subarray [7, -1, 2, 3] has the largest sum = 11.

Example 2
Input: arr = [-2, -4]
Output: -2

Explanation: Subarray [-2] has the largest sum = -2.

Approach – Kadane’s Algorithm

Initialize:
max_current = arr[0] → max sum ending at current position
max_global = arr[0] → max sum found so far
Traverse the array from index 1:
max_current = max(arr[i], max_current + arr[i]) → either start new subarray or extend existing
max_global = max(max_global, max_current) → update maximum found

Python Code

class Solution:
def maxSubarraySum(self, arr):
max_sum = arr[0]
curr_sum = 0

    for num in arr:
        curr_sum += num

        if curr_sum > max_sum:
            max_sum = curr_sum

        if curr_sum < 0:
            curr_sum = 0

    return max_sum

Output
Maximum Subarray Sum: 11
Maximum Subarray Sum: -2
Maximum Subarray Sum: 25