The short version
Both sites are built for the same moment: you have a data interview coming up and you want to practice the SQL questions that show up in it. They split on scope. DataLemur stays close to SQL and the analytics side of data science interviews. StrataScratch covers a wider loop, with Python, machine learning and AI engineering questions next to SQL, plus mock interviews and take-home projects.
So the first question isn't which site is better. It's which interview you're preparing for.
What DataLemur is built around
DataLemur is an interview question bank for data analyst and data science roles, made by the author of the book Ace the Data Science Interview. The question list mixes SQL with probability, statistics, machine learning and Python, and you can filter it by category and difficulty. Each question is tagged with the company it's attributed to.
SQL questions run in the browser on PostgreSQL by default, and the question we checked also let you switch to MySQL. The question page we opened showed a hint button and a written solution with more than one method, which is useful when you want to see the LEFT JOIN, NOT IN and NOT EXISTS versions of the same answer side by side.
If you're rusty, start with its free SQL tutorial for data analytics. It runs from SELECT and WHERE through GROUP BY, CASE, joins and date functions, then into CTEs, window functions, ranking, LEAD and LAG, self joins and pivoting.
The catch is on its pricing page: the paid plans are what give you the full set of SQL interview questions with hints and full solutions. So the free side covers part of the bank, not all of it. The top plan also adds a coaching call with the founder, which is a different kind of product from question practice.
What StrataScratch is built around
StrataScratch frames itself as interview prep for Data Science, Machine Learning and AI Engineering roles. Its question bank spans SQL, Python, ML system design and statistics, and the site leans into the newer parts of those loops:
- AI mock interviews. Timed sessions where an AI interviewer scores your code and your communication.
- Data Labs. Take-home style projects in cloud hosted Python notebooks, with no local setup.
- Free learning paths. A SQL path that goes from foundations through grouping, multi-table queries, subqueries and CTEs, date and text functions, and window functions, plus a matching Python path built on pandas.
- An AI coding tutor. Its blog announced a built-in tutor that explains questions, gives hints and debugs code.
You can start without a credit card, and a separate pricing page covers the paid parts. The tradeoff is focus. If your interview is a SQL screen for an analyst role, a lot of what StrataScratch offers is aimed at rounds you won't have.
How to choose between them
Pick based on the rounds in front of you, not on the size of the question bank.
- Analyst role, SQL screen. DataLemur's narrower focus helps. Work through its free tutorial, then its SQL questions, and stop there.
- Data scientist role with SQL and Python. StrataScratch covers both in one place, and its Python path is useful if pandas is the weak spot.
- ML or AI engineering role. StrataScratch is the closer fit, since its concept questions, take-home projects and mock interviews are aimed at those loops.
- No money to spend yet. Use the free parts of both before paying for either. DataLemur's tutorial and StrataScratch's learning paths are both free.
You don't need both subscriptions. Doing one set of questions properly beats having access to more of them.
When another option fits better
Sometimes neither is what you need.
HackerRank has a SQL practice area grouped into topics (Basic Select, Advanced Select, Aggregation, Basic Join, Advanced Join, Alternative Queries) with Easy, Medium and Hard filters. It works well for drilling fundamentals by topic, but its subdomain list doesn't include a dedicated window functions section.
PostgreSQL Exercises is free and runs real PostgreSQL. It goes from simple selects through joins, aggregates, dates, strings, window functions and recursive queries, all on one small dataset. That makes it good for Postgres syntax, less so for interview-style prompts.
Query Dojo is our site, so weigh this paragraph accordingly. Its practice questions run on real sample databases in your browser with no signup and no account, and every answer is checked against the expected result. Easy questions and the web-only practice pages are free; premium questions show the problem but keep the hints and solution for subscribers of the iPhone app, which also has timed mock interviews. It runs SQLite only, so it won't help you drill PostgreSQL or MySQL specifics. For a longer list of sites, see where to practice SQL.
A two-week plan with either site
- Days 1 to 3: patch the basics. Go through the free tutorial or learning path on the site you picked, and skip the lessons you can already do from memory.
- Days 4 to 7: core patterns. Do top N per group, second highest value, running totals and retention style questions. If you want a warm-up first, try Second Highest Salary and the window functions questions.
- Days 8 to 11: timed sets. Pick three or four questions you haven't seen, set a timer and talk through your approach before typing. A question like Monthly User Retention Rate is a fair test.
- Days 12 to 14: redo your misses. Go back to every question you got wrong and solve it again without looking at the old answer.
Habits that matter more than the site
- Write your own answer before opening hints. Reading a solution feels like progress, but it isn't the same skill.
- Test ties and NULLs on purpose. A query can pass on the sample rows and still break on duplicates or missing values.
- Say the query out loud. Explaining each step as you write it is practice for the interview itself, not only for the query.