Mastering SQL Through LeetCode: 20 Essential Questions

Here are the key SQL LeetCode questions shared by Shubham Wadekar to improve your SQL skills:

1. Combine Two Tables: https://lnkd.in/dumdCXC7
2. Employees Earning More Than Their Managers: https://lnkd.in/dKqcFmzY
3. Duplicate Emails: https://lnkd.in/dJSYfs89
4. Customers Who Never Order: https://lnkd.in/dPZPAKFf
5. Delete Duplicate Emails: https://lnkd.in/dEKnD9s3
6. Rising Temperature: https://lnkd.in/dyZvVYvP
7. Employee Bonus: https://lnkd.in/d9iTXt-V
8. Find Customer Referee: https://lnkd.in/d5cXjXdb
9. Big Countries: https://lnkd.in/dAnS54qK
10. Classes More Than 5 Students: https://lnkd.in/dfrUiutd
11. Sales Person: https://lnkd.in/dwRnf5Df
12. Triangle Judgement: https://lnkd.in/dwzD-hFn
13. Biggest Single Number: https://lnkd.in/d4F5zHDs
14. Not Boring Movies: https://lnkd.in/d5w8_z65
15. Swap Salary: https://lnkd.in/dnbCtecM
16. Product Sales Analysis 1: https://lnkd.in/dRj3EBuK
17. Project Employees 1: https://lnkd.in/dvGHsbKQ
18. Sales Analysis III: https://lnkd.in/d_y_rxPh

🔗 Additional Resources:

• Data Engineering Interview Experiences: https://lnkd.in/giY6RZu2
  https://youtube.com/ObhJ0xIwIsI

You Should Know: SQL & Data Engineering Commands

SQL Commands for Practice

-- Example: Combine Two Tables (LeetCode 175) 
SELECT p.firstName, p.lastName, a.city, a.state 
FROM Person p 
LEFT JOIN Address a ON p.personId = a.personId;

-- Employees Earning More Than Managers (LeetCode 181) 
SELECT e1.name AS Employee 
FROM Employee e1 
JOIN Employee e2 ON e1.managerId = e2.id 
WHERE e1.salary > e2.salary;

-- Find Duplicate Emails (LeetCode 182) 
SELECT email 
FROM Person 
GROUP BY email 
HAVING COUNT(email) > 1; 

Linux & Data Engineering Commands

 Extract data from PostgreSQL to CSV 
psql -U username -d dbname -c "SELECT  FROM table" -o output.csv

Process CSV with Python (Pandas) 
python3 -c "import pandas as pd; df = pd.read_csv('output.csv'); print(df.head())"

Automate SQL queries with cron 
crontab -e 
 Add: 0 3    /usr/bin/pg_dump -U user dbname > backup.sql 

Windows PowerShell for Data Handling

 Export SQL Server data to CSV 
Invoke-Sqlcmd -Query "SELECT  FROM Employees" -ServerInstance "SQLSERVER" | Export-Csv -Path "employees.csv"

Bulk insert into SQL Server 
bcp Database.dbo.Table IN "data.csv" -T -c -t "," -S ServerName 

What Undercode Say

SQL mastery is essential for data engineers, analysts, and cybersecurity professionals. These LeetCode problems sharpen query optimization, joins, and subqueries—key for database security (SQL injection defense) and performance tuning.

Key Takeaways:

✔ JOINs are crucial for merging datasets.

✔ Aggregations (GROUP BY, HAVING) help detect anomalies.

✔ Subqueries optimize complex filtering.

Advanced SQL for Security:

-- Prevent SQL injection with parameterized queries (Python) 
cursor.execute("SELECT  FROM users WHERE username = %s", (user_input,))

-- Audit database access (PostgreSQL) 
CREATE ROLE read_only; 
GRANT SELECT ON ALL TABLES TO read_only; 

Expected Output:

A structured, hands-on approach to SQL that bridges theory (LeetCode) and real-world data engineering tasks.

Prediction

As AI-driven data pipelines grow, SQL will remain foundational. Expect more automated query optimization tools (like AI-powered SQL assistants) but core query-writing skills will stay in demand.

🔗 Further Learning:

• PostgreSQL Docs: https://www.postgresql.org/docs/
• SQL Injection Prevention: OWASP Guide

References:

Reported By: Shubhamwadekar I – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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