🧠 Advanced SQL Interview Question ⚡

📊 Find customers who placed orders in every month of 2025

Table: Orders

customer_id | order_date

🔍 Query:

SELECT customer_id
FROM Orders
WHERE YEAR(order_date) = 2025
GROUP BY customer_id
HAVING COUNT(DISTINCT MONTH(order_date)) = 12;

🎯 Why this question matters:

✅ Tests GROUP BY + HAVING concepts
✅ Uses DISTINCT counting logic
✅ Evaluates date function knowledge

🚀 Pro Tip:

✅ COUNT(DISTINCT ...) is commonly used in retention & activity analysis

✅ Month-wise activity questions are very common in analytics interviews

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