🔥 SQL Interview Case Studies & Real-World Business Problems

🧠 Case Study 1: Top 3 Customers by Revenue
📊 Orders Table
order_id customer_id amount
1 101 500
2 102 1000
3 101 700

❓ Business Question
Find the top 3 customers by total revenue.

✅ Solution
SELECT customer_id,
SUM(amount) AS total_revenue
FROM orders
GROUP BY customer_id
ORDER BY total_revenue DESC
LIMIT 3;

🧠 Case Study 2: Department with Highest Average Salary

❓ Business Question
Which department has the highest average salary?

✅ Solution
SELECT department,
AVG(salary) AS avg_salary
FROM employees
GROUP BY department
ORDER BY avg_salary DESC
LIMIT 1;

🧠 Case Study 3: Customers Who Never Ordered
📊 Tables
Customers customer_id name
Orders order_id customer_id

❓ Business Question
Find customers who never placed an order.

✅ Solution
SELECT c.customer_id,
c.name
FROM customers c
LEFT JOIN orders o
ON c.customer_id = o.customer_id
WHERE o.customer_id IS NULL;

🧠 Case Study 4: Second Highest Salary

❓ Business Question
Find employees with the second highest salary.

✅ Solution
SELECT *
FROM employees
WHERE salary = (
SELECT MAX(salary)
FROM employees
WHERE salary < (
SELECT MAX(salary)
FROM employees
)
);

🧠 Case Study 5: Monthly Sales Trend

❓ Business Question
Calculate monthly sales.

✅ Solution
SELECT YEAR(order_date) AS year,
MONTH(order_date) AS month,
SUM(amount) AS sales
FROM orders
GROUP BY YEAR(order_date),
MONTH(order_date)
ORDER BY year, month;

🎯 Practice Tasks
1️⃣ Find top-selling product
2️⃣ Find employee with highest salary in each department
3️⃣ Find customers with more than 5 orders
4️⃣ Find month with highest sales
5️⃣ Find departments having more than 10 employees

⚡ Mini Challenge 🔥
E-commerce Scenario

Tables:
Customers customer_id name
Orders order_id customer_id amount order_date

Business Question
Find the top 5 customers by total spending in the last 12 months.

🔥 Interview Tip
Most SQL interviews are NOT about syntax.

They're about:
✅ Understanding business problem
✅ Choosing the right approach
✅ Writing efficient SQL

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