SQL Query Semantics vs Quarry

SQL Query Semantics is an independent study app for understanding how SQL queries really work. Use it to build confidence with relational logic, joins, grouping, null handling, predicates, subqueries, and result-set reasoning. Designed for students, developers, data analysts, and interview candidates, the app includes 160 focused practice questions across Fundamentals, Practical Skills, Common Pitfalls, and Advanced Concepts. Features: - FSRS spaced-repetition intervals for smarter review - Weak-point heatmap to spot topics that need work - Daily challenge for consistent practice - Mock exams for timed review - Explanations for every question - Offline practice with no login required Practice offline, no login required. Independent study app for SQL Query Semantics practice. Not affiliated with, endorsed by, or sponsored by any company, certification body, exam operator, or vendor. All trademarks belong to their respective owners.

Data interview practice that runs entirely on your device. Write real SQL and Python (pandas), get graded instantly, and see exactly why. NEW IN 1.1 • Designed for iPhone Duo: question and editor side by side when open, a focused layout when closed, and it adapts as you fold. • A new native design for iPhone and iPad, in light and dark mode. • 60 new problems: a staff-level data engineering set (gaps and islands, sessionisation, slowly changing dimensions, funnels, retention) and company-style SQL sets. • Clearer results: instant correct / not quite feedback, expected vs. actual tables, and the query plan for your SQL. WHAT'S INSIDE • 168 problems across three tracks: 61 SQL, 22 pandas and 85 Python (algorithms and data structures). • Real execution, real grading. Your output is checked against the expected result, with no self-marking. • Company-style practice. Filter problems written in the style of the company you're interviewing at. • Hints and full solutions when you're stuck. • Difficulty levels from Easy to Hard, grouped by topic. • A free-form Playground for scratch SQL and Python. PRIVATE BY DESIGN No accounts. No analytics. No ads. No tracking. Quarry collects nothing and works fully offline. Built by a working data engineer, for analysts, data engineers, data scientists and software engineers who want a quiet, fast place to actually practice.