🧠 SQL Interview Question (Moderate–Tricky & Duplicate Transaction Detection)
📌

transactions(transaction_id, user_id, transaction_date, amount)

❓ Ques :

👉 Find users who made multiple transactions with the same amount consecutively.

🧩 How Interviewers Expect You to Think

• Sort transactions chronologically for each user
• Compare the current transaction amount with the previous one
• Use a window function to detect consecutive duplicates

💡 SQL Solution

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

🔥 Why This Question Is Powerful

• Tests understanding of LAG() for row comparison
• Evaluates ability to identify patterns in sequential data
• Reflects real-world use cases like detecting suspicious or duplicate transactions

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✅ Coding Interview Prep Guide 💻🔥

1️⃣ Core Programming Fundamentals
• Variables, data types, operators
• Control flow (loops, conditions)
• Functions recursion
• Time space complexity basics
• Debugging mindset

2️⃣ Data Structures (High Priority)
• Arrays Strings
• Linked Lists
• Stacks Queues
• HashMaps / Dictionaries
• Trees Binary Trees
• Heaps Priority Queues
• Graphs (BFS, DFS)

3️⃣ Algorithms You MUST Know
• Searching (Binary Search)
• Sorting (Quick, Merge, Heap)
• Recursion Backtracking
• Greedy algorithms
• Dynamic Programming
• Sliding Window
• Two Pointers
• Prefix Sum

4️⃣ Problem-Solving Patterns
• Brute force → optimized approach
• Hashing for lookups
• Divide and conquer
• Recursion → DP conversion
• Space–time tradeoffs

5️⃣ Language-Specific Prep
• Python / Java / C++ fundamentals
• Built-in data structures
• Edge cases constraints
• Writing clean, readable code
• Input/output handling

6️⃣ Coding Interview Expectations
• Explain approach before coding
• Write code step-by-step
• Handle edge cases
• Analyze time space complexity
• Optimize if asked

7️⃣ Common Interview Questions
• Reverse a string / array
• Find duplicates
• Two Sum / Subarray problems
• Palindrome checks
• Tree traversal
• LRU Cache
• Longest substring problems

8️⃣ Where to Practice
• LeetCode (Top priority)
• HackerRank
• Codeforces
• CodeChef
• GeeksforGeeks

9️⃣ Mock Interview Focus
• Think out loud
• Don’t panic on hard questions
• Ask clarifying questions
• Partial solutions still matter
• Correct approach > perfect code

🔟 Pro Tips
✔️ Master patterns, not random problems
✔️ Revise mistakes weekly
✔️ Practice writing code without IDE help
✔️ Speed improves with consistency
✔️ Interviews test thinking, not memory

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