🧠 SQL Interview Question (Moderate–Tricky & Identifying Users with Increasing Transactions)
📌

transactions(transaction_id, user_id, transaction_date, amount)

❓ Ques :

👉 Find users whose transaction amount strictly increases with every new transaction.

🧩 How Interviewers Expect You to Think

• Sort transactions by date for each user
• Compare each amount with the previous one
• Identify users whose amounts always increase

💡 SQL Solution

WITH t AS (
SELECT
user_id,
amount,
LAG(amount) OVER (
PARTITION BY user_id
ORDER BY transaction_date
) AS prev_amount
FROM transactions
)

SELECT user_id
FROM t
GROUP BY user_id
HAVING SUM(
CASE
WHEN prev_amount IS NOT NULL AND amount <= prev_amount
THEN 1 ELSE 0
END
) = 0;

🔥 Why This Question Is Powerful

• Tests understanding of LAG() with conditional logic
• Evaluates ability to validate patterns across sequential data
• Reflects real-world analytics like tracking user spending growth trends

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