SQL Interview Questions — Basics
SELECT, filtering, joins and aggregation questions with explanations.
Reasoning questions
Explain the business grain before writing the solution.
A strong answer states what one row represents and which key should be unique.
How would you validate the result?
Use a known sample, row counts, duplicates, missing keys and a manual reconciliation.
What edge case would you test?
Missing values, duplicate keys, zero denominators, type mismatches and unexpected one-to-many relationships.
Practical task
You receive a small operational dataset with one deliberate duplicate, one missing value and one unmatched key. Explain how you would profile it, solve the requested task and prove the result is correct before presenting it.
Open challenge library