SQL interview questions
Practice explaining your reasoning, checking assumptions, and supporting your answers with real examples.
Prepare to show your thinking
These sql questions are practice prompts, not a prediction of a particular company’s interview. A strong answer explains your reasoning and adapts to the interviewer’s constraints. Start by clarifying the question and state assumptions before proposing a solution.
Use examples from work or projects you actually completed. If you have not encountered a situation, say so and explain how you would approach it. Do not turn a hypothetical answer into a fabricated personal story.
How can a join accidentally multiply rows?
Discuss key cardinality, duplicate matches, and how to verify assumptions before aggregating.
Practice aloud for two minutes, then ask yourself which assumptions you left unstated. A follow-up to prepare for: what would make you choose a different approach?
How would you investigate a slow query?
Use the execution plan, row estimates, indexes, filters, and realistic data volumes. Avoid assuming every query needs a new index.
Use one concrete example and explain how you would verify that your answer works. A follow-up to prepare for: what failure case have you not yet considered?
When would a transaction be necessary?
Explain a multi-step update with an invariant, possible partial failure, and an appropriate isolation tradeoff.
Distinguish the ideal design from what is reasonable under time and resource constraints. A follow-up to prepare for: how would you explain this choice to a colleague?
Review your practice answer
Record or write a short answer, then check four things: did you answer the actual question, make your assumptions visible, support the key claim, and explain a tradeoff? Revise the structure rather than memorizing a script.
Prepare one question of your own about how the team approaches this work. An interview can help you understand expectations as well as demonstrate your skills.
Questions worth asking
Should I memorize sample answers?+
No. Practice a structure and use real evidence. Memorized scripts make it harder to respond when the interviewer changes the constraints.
What if I do not know the answer?+
Say what you know, identify the gap, and explain how you would investigate. Ask a clarifying question and avoid pretending certainty.