🔹 SQL Example
BEGIN;
UPDATE accounts
SET balance = balance - 100
WHERE id = 1;
ROLLBACK;

🔹 Interview Tip
Transactions protect data integrity.

🚀 78. What is connection pooling?
Opening DB connections repeatedly is expensive.
Connection pooling: Reuses existing connections

🔹 Flow
App → Connection Pool → Database

🔹 Benefits
✅ Faster performance
✅ Reduced overhead
✅ Better scalability

🔹 Popular Tools
• HikariCP
• PgBouncer

🔹 Interview Tip
Connection pools are critical in high-traffic backend systems.

🚀 79. What is the CAP theorem?
CAP theorem states distributed systems can only guarantee TWO of:

🔹 CAP

C
• Meaning: Consistency

A
• Meaning: Availability

P
• Meaning: Partition Tolerance

🔹 Explanation

🔹 Consistency
All nodes return same data.

🔹 Availability
System always responds.

🔹 Partition Tolerance
System survives network failures.

🔹 Reality
In distributed systems: Partition tolerance is mandatory
So trade-off becomes: Consistency vs Availability

🔹 Interview Tip
CAP theorem is fundamental for distributed systems interviews.

🚀 80. How do you design a scalable schema for user-generated content?
Examples:
• Social media posts
• Comments
• Reviews
• Videos

🔹 Core Tables

🔹 Users
• user_id
• name

🔹 Posts
• post_id
• user_id
• content

🔹 Comments
• comment_id
• post_id
• user_id

🔹 Scalability Techniques
✅ Indexes
✅ Caching
✅ CDN for media
✅ Database sharding
✅ Async processing

🔹 Media Storage
Store images/videos in:
• Amazon Web Services S3
• Object storage systems

🔹 Feed Optimization
Use: Precomputed feeds for faster timeline generation.

🔹 Interview Tip
Scalable schema design focuses on:
• Read efficiency
• Write scalability
• High traffic handling

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🧠 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;

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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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