Canonical: this is a cross-post. The original lives at https://four-leaf.ai/blog/sql-interview-guide
Key takeaways
A SQL interview is a translation test. The syntax is the easy half, and most candidates who fail were fluent in SQL. What sinks them is starting to type before the question is pinned down, or writing a correct query that answers something nobody asked. The round scores three things: whether you clarify before you write, whether your query is correct on the edge cases, and whether you can explain your reasoning while your hands are moving. It also looks meaningfully different depending on whether you're interviewing as a data analyst, a data scientist, or an engineer.
What does a SQL interview test?
The round tests whether you can turn an underspecified business question into a query that survives contact with real data. Interviewers rarely hand you a clean specification, because the job never does either.
SQL is worth preparing for on volume alone. The Stack Overflow 2025 Developer Survey found 58.6% of developers had used SQL in the past year, making it the third most-used language behind JavaScript at 66% and HTML/CSS at 61.9%, and ahead of Python at 57.9%. It shows up in loops for roles that aren't nominally data roles at all, which is part of why candidates underprepare for it.
The tell that separates a strong candidate is what happens in the first sixty seconds. Weak candidates read the prompt and start typing. Strong candidates restate the question, ask what counts as an active user or a completed order, confirm whether the answer should include rows with nulls, and only then write. That opening exchange is often worth more to the interviewer than the query itself, because it's the part of the job that can't be looked up.
What kinds of SQL questions come up?
Three shapes cover most of what gets asked, and they escalate in a predictable order.
The first is a join and aggregation question. Given two or three tables, produce a count or a sum grouped by something. These look trivial and catch people on join type, because an inner join silently drops the users who never ordered, and the question usually wanted them counted as zero.
The second is a window function question. Rank purchases per customer, find each user's second transaction, compute a running total, or calculate a month-over-month change. This is where a lot of loops separate candidates, because window functions are the boundary between people who write SQL occasionally and people who use it as a primary tool.
The third is an open business question against a schema you've just been shown. "Tell me whether retention improved after the March release." There's no single correct query. The interviewer is watching how you decompose the question, what you decide retention means, and whether you notice that the March cohort has less time to churn than the February one.
How is a SQL interview scored?
Three dimensions, and they map to those question shapes. It helps to run each one against the questions the round keeps returning to: give me a count by group, rank something per user, and answer this vague business question.
Do you clarify before you write? A passing answer asks one question about the schema. A strong answer names the ambiguity that would change the query and resolves it. "Does an active user mean any event in the window, or a purchase?" is the difference between two very different numbers, and interviewers plant that ambiguity on purpose.
Is your query correct on the edges? A passing answer runs. A strong answer accounts for nulls, duplicates, ties in a ranking, and rows on the boundary of a date range. Say the edge case out loud when you handle it, because a silent LEFT JOIN looks identical to a lucky one.
Can you narrate while you write? A passing answer goes quiet and produces a query. A strong answer talks through the plan first, writes, then checks the result against a rough expectation out loud. Interviewers score reasoning they can hear, and a correct query delivered in silence gets a weaker write-up than a slightly imperfect one that was explained.
How does a SQL interview differ for a data analyst versus a data scientist?
Same language, meaningfully different round, and mixing them up is the most common preparation mistake in the function.
Data analyst. The emphasis is breadth and business translation. Expect more questions, less depth per question, and heavy weighting on whether your numbers would hold up in front of a stakeholder. Definitional precision matters more than query elegance. You're likely to be asked what a metric should mean before you're asked to compute it, and an answer that flags a misleading denominator scores higher than one that optimizes a join.
Data scientist. The emphasis shifts toward experiment and cohort logic. Expect fewer questions with more depth, and expect at least one that touches an A/B test readout, a cohort definition, or a sampling problem hiding inside the SQL. A question about whether a March cohort had less time to churn than a February one is a data scientist question. Correctness under a statistical framing is what's being scored, not query volume.
Analytics engineer. The round moves toward modeling. Expect questions about how you'd structure the model rather than write a one-off answer, including idempotency, incremental logic, and what happens when the query is rerun.
Software engineer. SQL usually appears inside a broader technical round rather than on its own, and the framing is performance and correctness at scale. Expect indexing, query plans, and the consequences of a full table scan rather than business definitions.
Product management. Increasingly common and usually light. The bar is whether you can pull your own numbers without asking an analyst, so expect one aggregation question and no window functions.
Four-Leaf covers the wider data loop in how to prepare for a data science interview.
What is overrated
Memorizing exotic syntax. Recursive CTEs and pivot tricks appear in practice sets far more often than in interviews, and time spent there is time not spent on window functions, which appear constantly.
Speed is the other overrated thing. Candidates rush because they assume the clock is the test, then miss a join type. The interviewer is almost always more interested in whether you caught the null case than in whether you finished ninety seconds early.
Using an AI assistant to get through this round is a losing trade. CodeSignal reported in February 2026 that cheating and fraud attempts on proctored assessments more than doubled, from 16% in 2024 to 35% in 2025. Interviewers noticed. A 2025 interviewing.io survey of 67 of them, 52 at FAANG companies, found 81% suspected candidates of using AI and about a third had caught someone at it. The visible response has been more live, narrated rounds, which is precisely the format where a candidate who can't explain their own query falls apart.
A five-step playbook for a SQL round
-
Drill window functions until they're automatic.
ROW_NUMBER,RANK,DENSE_RANK,LAG,LEAD, and a running sum. These carry more interview weight per hour of study than anything else in SQL. - Practice restating the question out loud before writing. Give yourself a rule that you don't type until you've named one ambiguity.
- Build a null and duplicate checklist. Before you call a query done, ask what happens to rows with nulls, rows that appear twice, and rows exactly on the boundary date.
- Do at least three problems on a whiteboard or plain text editor. Autocomplete hides gaps that a bare editor exposes, and many live rounds use a bare editor.
- Narrate a solved problem end to end. Explaining a query you already understand is a separate skill from writing it, and it's the one being scored. A voice mock interview is a reasonable way to rehearse the narration, since the failure mode is going quiet under pressure rather than not knowing the syntax.
Where this is heading
SQL rounds are getting more conversational, and the reason is the same force reshaping every technical loop. As assistants make it trivial to produce a syntactically correct query, interviewers move the bar to the part that's harder to fake, which is deciding what to compute and defending the choice. Four-Leaf's AI-Era Hiring Index, a study of 3,502 open roles at 16 top tech employers, found LLM or foundation-model experience listed in 57% of data and machine learning job descriptions against 21% of engineering ones, so the expectation that data candidates work alongside these tools is already written into the postings.
That direction rewards the candidate who understood the question. The syntax was never the hard part, and it's about to matter even less. Where this round sits in the wider sequence is covered in how an onsite interview loop actually works. To rehearse it out loud rather than read about it, start with eight coding interview platforms, compared.
