𝗜𝗻𝘁𝗲𝗿𝘃𝗶𝗲𝘄𝗲𝗿:
You have 2 minutes to solve this SQL query.

Q: Find the customer(s) who placed orders in every month of the year 2025.

Assume the table structure:
orders(order_id, customer_id, order_date)

𝗠𝗲: Challenge accepted! 💪

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

💡 Explanation:
This query identifies customers who placed at least one order in every month of 2025.

• WHERE YEAR(order_date) = 2025 filters orders from the year 2025
• GROUP BY customer_id groups all orders by customer
• COUNT(DISTINCT MONTH(order_date)) counts the unique months in which each customer placed an order
• HAVING ... = 12 ensures the customer has orders in all 12 months

This question tests your understanding of:
✅ Date Functions (YEAR, MONTH)
✅ GROUP BY
✅ HAVING
✅ COUNT(DISTINCT)

🎯 Expected Output Example

| Customer ID |
|-------------|
| 101 |
| 205 |

These customers placed at least one order in every month of 2025.

🚀 Alternative (Database-Agnostic SQL)

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

This version works with databases like PostgreSQL and Oracle that support the EXTRACT() function.

🚀 Tip for SQL Job Seekers:
Whenever you see interview questions containing phrases like:
"Every month" / "Every quarter" / "Every year" / "Every category"

Think of COUNT(DISTINCT ...) combined with GROUP BY and HAVING. This is a very common SQL interview pattern.

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