Deloitte Interview Question — Data Engineer III

🧩 Fixing a Reversed Column — A Classic SQL & PySpark Interview Puzzle

Mohit Daxini

3 min read

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4 days ago

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Imagine this 👇
You’re a Data Engineer. An Excel file was loaded into a SQL database… but something went wrong.
UserID ✅ loaded correctly
FirstName ✅ loaded correctly
LastName ❌ loaded in reverse order

Friend link for non-member readers is here — Link

Your mission (should you choose to accept it 😄):

👉 Fix the LastName column so each user gets their correct last name — without reloading the data.

This is a very popular interview question (Deloitte, Accenture, Big4 vibes 😄), and it tests how well you understand ordering, window functions, and data transformations.

Let’s solve it step by step, first in SQL, then in PySpark, in a way that even beginners can follow.

🧠 Understanding the Problem (Beginner Friendly)
Input Table (Incorrect Data)

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