Practice-first SQL repository · Built for long-term interview readiness

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✨ Overview

A personal SQL practice bank designed to retain fluency through continuous hands-on problem solving.

This repository does not teach SQL concepts.
It exists to practice them — the same way LeetCode problems strengthen logic after learning fundamentals.

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🧭 Practice Philosophy

Step	Action
1️⃣	Revise SQL fundamentals (notes / docs / videos)
2️⃣	Open a .sql file from this repo
3️⃣	Comment out the solution
4️⃣	Solve the problem yourself
5️⃣	Compare & optimize

Each file is a problem sheet, not a walkthrough.

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🧠 Quick Recall — GROUP BY & Aggregations

One-glance rules to reset your brain before writing queries

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✅ GROUP BY Rules (Simple & Sharp)

• All non-aggregated columns in SELECT must appear in GROUP BY
  (but not all GROUP BY columns must appear in SELECT)

• Columns inside aggregate functions do NOT need to be in GROUP BY

• You cannot use aggregate functions in WHERE
  → use HAVING instead

• Query execution order: FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY

• Each group produces exactly one row in the result

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✅ Aggregation Rules

• Aggregate functions return one value per group

• Aggregate functions ignore NULL values
  (except COUNT(*))

• You cannot mix aggregated and non-aggregated columns in SELECT
  without GROUP BY

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🧠 One-line Memory Trick

SELECT = GROUP BY columns + aggregate functions only

Never use GROUP BY in the same SELECT where you use window functions on raw columns

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⚙️ Environment

Tool	Usage
🗄️ Database	MySQL
🧪 Execution	MySQL Workbench / compatible clients
🎯 Focus	Query logic, edge cases, interview thinking

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📚 Topics Covered

🟢 Core SQL

• SELECT, DISTINCT
• WHERE clause
• ORDER BY, LIMIT, OFFSET
• Filtering using IN, BETWEEN, LIKE, IS NULL
• Conditional filtering patterns

🔵 Aggregations

• COUNT, SUM, AVG, MIN, MAX
• GROUP BY
• HAVING
• Conditional aggregations
• Multi-column grouping logic

🟣 Joins (Interview Heavy)

• INNER JOIN
• LEFT JOIN, RIGHT JOIN
• SELF JOIN
• Multi-table joins
• Join conditions vs filter conditions
• Handling NULLs in joins

🟠 Subqueries

• Subqueries in SELECT, WHERE, and FROM
• Nested subqueries
• Correlated subqueries
• EXISTS / NOT EXISTS
• Subqueries vs joins

🔴 Window Functions

• ROW_NUMBER()
• RANK() / DENSE_RANK()
• PARTITION BY
• Ranking and ordering logic
• Nth highest value problems
• Top-N per group queries

🟡 Conditional Logic

• CASE WHEN
• Conditional aggregations
• Multi-condition branching
• Categorization and bucketing problems

🟤 Date & Time

• Date filtering
• Time-based grouping
• Difference between dates
• Month / year level aggregations
• Edge cases in date logic

⚫ Advanced Patterns

• Duplicate detection
• Consecutive records
• Gap analysis
• Rolling calculations
• Multi-step query breakdowns
• Interview-driven logic building

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📂 Repository Design

✅ Practice-oriented queries
✅ No concept explanations
✅ Solutions included only as reference
✅ Optimized for re-solving

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🎯 Why This Exists

• Maintain long-term SQL muscle memory
• Avoid last-minute interview panic
• Improve query speed & accuracy
• Practice real-world interview logic

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🔗 Reference & Credits

This repository was built using publicly available learning material for practice and problem exposure.

Special reference:

• The Grand Complete Data Science Materials – MySQL Interview Section
  https://github.com/krishnaik06/The-Grand-Complete-Data-Science-Materials/tree/main/Complete%20MySQL%20Interview%20Materials

All problems are re-solved independently and organized for long-term interview preparation.

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🧠 Final Thought

“SQL is not remembered — it is retrained.”