Day 22/30 - DEEP DIVE SOLUTIONS: SQL, PYTHON, ETL, DATA MODELING

Solutions for March 25th, 2025 Challenge – Full Breakdown + Live Runnable Code

Hello Data Engineers,

Today’s deep dive explores nested subqueries, dictionary comprehensions, incremental ETL logic, and denormalization strategies. These concepts show up repeatedly in interviews and production systems. Let’s break them down clearly and practically.

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SQL Deep Dive: Filtering with Nested Subqueries

Challenge Recap:
❓ What will be the output of the following SQL query?

SELECT name  
FROM employees  
WHERE salary > (  
  SELECT AVG(salary)  
  FROM employees  
);

🔘 A) Employees with the lowest salary
🔘 B) All employees
🔘 C) Employees earning above average salary
🔘 D) SQL Error

✅ Answer: Option C - Employees earning above average salary

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Why This Happens:

The subquery (SELECT AVG(salary) FROM employees) runs first and returns a scalar value. The outer query then filters rows where salary > average.

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Where It’s Used in Real-World Applications:

• Showing top performers by comparing against global averages

• Filtering rows based on rolling metrics

• Salary band analysis or anomaly detection