Data Engineering for Beginners

APPENDIXSample Interview Questions

In this chapter, we’ll walk through sample interview questions that cover some of the most important areas in data engineering interviews. Think of this appendix as a set of flashcards, something you can quickly flip through when preparing for interviews to refresh your memory and sharpen your thinking. Use it to test yourself, spot any weak areas, and build the confidence you need to walk into your interviews.

SQL

What is the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN?

INNER JOIN returns rows when there’s a match in both tables. LEFT JOIN returns all rows from the left table and matches rows from the right. RIGHT JOIN does the opposite. FULL OUTER JOIN returns all rows from both tables, matching where possible and filling in NULLs where there’s no match.

What are indexes, and how do they improve performance?

Indexes are special lookup tables that the database uses to speed up data retrieval. They allow queries to find rows faster, especially in large datasets, but can slow down inserts and updates due to maintenance overhead.

What is the difference between WHERE and HAVING clauses?

WHERE filters rows before grouping, whereas HAVING filters groups after aggregation. Use WHERE for raw data filtering and HAVING when dealing with GROUP BY queries.

What does GROUP BY do?

GROUP BY aggregates rows that have the same values in specified columns, allowing you to perform aggregate functions like SUM, COUNT, or AVG