SQL-Problem-Solving-Series

This repository contains a series of SQL practice problems and solutions designed to help learners strengthen their SQL skills. It covers a wide range of topics from basic queries to advanced SQL concepts.

Topics Covered

• Basic SELECT queries
• Filtering using WHERE conditions
• Aggregate functions: COUNT, SUM, AVG, MIN, MAX
• String functions: CONCAT, UPPER, LOWER, SUBSTRING
• Numeric functions: ROUND, CEIL, FLOOR, ABS
• Date and time functions: NOW, CURDATE, DATEDIFF
• Conditional functions: CASE, COALESCE, NULLIF
• Sorting with ORDER BY and limiting rows using LIMIT/OFFSET
• Grouping data using GROUP BY and HAVING
• Joins: INNER JOIN, LEFT JOIN, RIGHT JOIN, SELF JOIN
• Ranking and window functions: ROW_NUMBER(), RANK(), DENSE_RANK()

Purpose

The goal of this repository is to provide a structured way to practice and master SQL through real-life inspired problems. Each problem includes:

1. A description of the problem
2. Sample data or tables
3. SQL query solution with explanations and comments

This is ideal for students, developers, and anyone preparing for SQL interviews or competitive coding challenges.

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🔹 Easy (Basic Queries & Simple Joins)

1. Count total employees (COUNT(*))
2. Find highest salary (MAX())
3. Find lowest salary (MIN())
4. Find average salary (AVG())
5. List employees hired after a specific date (WHERE + >=)
6. Display employee names in uppercase (UPPER())
7. Display employee names in lowercase (LOWER())
8. List first 5 employees (LIMIT)
9. Find employees in a specific department (WHERE Department = 'IT')
10. Show distinct departments (DISTINCT)
11. Find employees with NULL manager (IS NULL)
12. Find employees whose salary is above 50,000 (WHERE Salary > 50000)
13. Employees whose name starts with a specific letter (LIKE 'A%')
14. Employees whose name contains a substring (LIKE '%an%')
15. Simple join: Show employees with their department names (INNER JOIN Departments)

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🔹 Medium (Aggregates, Grouping, Joins, Subqueries)

1. Count employees per department (GROUP BY)
2. Total salary per department (SUM(Salary) GROUP BY Department)
3. Average salary per department (AVG(Salary) GROUP BY Department)
4. Find 2nd highest salary (MAX() + subquery)
5. Find 3rd highest salary (MAX() + nested subqueries)
6. Find employees with salary greater than department average (JOIN or subquery)
7. Top 3 highest salaries (ORDER BY Salary DESC LIMIT 3)
8. Count employees joined after 2020 per department (GROUP BY + WHERE)
9. Employees who belong to multiple departments (JOIN or GROUP_CONCAT equivalent)
10. Employees not in a specific department (WHERE NOT IN or LEFT JOIN IS NULL)
11. Show employee with their manager name (SELF JOIN)
12. Show employees and their department along with department head (JOIN)
13. Employees whose salary is above average using a subquery
14. Department-wise highest and lowest salary (GROUP BY + MAX/MIN)
15. Rank employees by salary using window function (ROW_NUMBER() / RANK() if supported)

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✅ These 30 problems are a good mix of:

• Easy → simple filters, aggregates, string functions, simple joins.
• Medium → grouping, subqueries, multiple joins, ranking, and commonly asked SQL interview questions.