🧠 SQL Interview Question (Detect Negative Account Balance)
📌

transactions(txn_id, txn_date, amount)
(credit = +ve, debit = -ve)

❓ Ques :

👉 Find the first date when account balance becomes negative

👉 Return txn_date

🧩 How Interviewers Expect You to Think

• Calculate running balance over time 💰
• Use cumulative sum
• Track when balance drops below zero
• Return first occurrence

💡 SQL Solution

WITH balance_cte AS (
SELECT
txn_date,
SUM(amount) OVER (
ORDER BY txn_date
) AS running_balance
FROM transactions
)

SELECT txn_date
FROM balance_cte
WHERE running_balance < 0
ORDER BY txn_date
LIMIT 1;

🔥 Why This Question Is Powerful

• Tests cumulative sum (window function) 🧠
• Very common in fintech & transaction analysis
• Checks real-world problem solving ability

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