SQLDOU vs Learn SQL

SQLDOU is an educational app that visualizes SQL query execution plans with step by step animation. The third entry in the DOU series, following PAKEDOU (networking) and KURAUDOU (cloud). This time the theme is SQL. Enter a query and watch the execution plan generate automatically, then follow each row as it flows through filtering, sorting, joining and finally arrives at the result. WHAT YOU WILL LEARN • Full Scan vs Index Why scanning every row (full scan) is slow. See how much faster a B Tree index makes searches. Run the same SQL with both scan methods and compare speed in real time. • B Tree Index O(log n) Supports range queries such as greater than, less than, greater than or equal, less than or equal and equality. Watch the tree structure locate target rows efficiently through animation. • Hash Index O(1) Pro A high speed index specialized for equality searches. Experience the real execution time difference between Hash and B Tree. • WHERE and DISTINCT See each row pass or get skipped by the WHERE condition in real time with PASS or SKIP badges. Observe DISTINCT removing duplicates step by step. • JOIN Pro Visualize how rows from two tables are combined. See the batch process that waits for both sides before executing the join. • GROUP BY and ORDER BY Pro Watch aggregation such as COUNT, SUM, AVG, MAX and MIN and sorting such as ASC and DESC animate their internal processing. Observe how they buffer all rows before producing output. • Custom Quiz Mode v3.0 Pro Define your own table name, columns and rows, write any SQL you want, and save the quiz. Replay or edit later. Go beyond textbook examples and reproduce real world scenarios you care about. 20 MISSIONS Progress through 20 missions across 6 levels. • Level 1 SELECT and WHERE 5 missions SQL fundamentals • Level 2 Scan Methods 4 missions Full scan vs index speed comparison • Level 3 ORDER BY and DISTINCT 3 missions Pro • Level 4 GROUP BY 3 missions Pro • Level 5 JOIN 2 missions Pro • Level 6 Final Challenge 3 missions Index optimization Pro Each mission includes a prediction quiz. The Predict Execute Verify cycle encourages active learning. Star ratings up to three track your progress. EDUCATIONAL FEEDBACK • Auto comparison run the same SQL with full scan and index and see the speed difference automatically • Scan narration watch each row being checked in real time with PASS or SKIP verdict • Diagnostic comments context aware explanations Pro Why slow How to improve Background knowledge • Table data preview see actual data while making predictions FREE AND PRO Free • Level 1 and Level 2 missions total 9 missions • Basic diagnostics one line comment • B Tree index • Free mode Pro one time purchase • All 20 missions unlocked • Detailed diagnostics Why How to improve Background • Hash index support • Advanced SQL JOIN GROUP BY ORDER BY DISTINCT • Custom Quiz Mode build save and run your own quizzes • No ads WHO IS THIS FOR • Students and junior engineers learning SQL basics • Anyone who wants to intuitively understand why indexes are fast • Database beginners learning execution plans • Instructors looking for training supplements • Developers building a foundation in SQL performance tuning

Let Us SQL is a learn-anywhere app that takes you from zero to confident with SQL, the language behind every database. No prior coding experience needed — you'll learn by doing, one bite-sized lesson at a time, until reading and writing SQL feels like second nature. Best of all, everything is taught using PostgreSQL, one of the most widely used databases in the real world, so the skills you build here are skills you'll actually use. 85 LESSONS. 11 CHAPTERS. A CLEAR PATH FROM BEGINNER TO JOB-READY. We've laid out a complete, carefully sequenced curriculum: 11 chapters and 85 lessons in total. You'll start with your very first SELECT, then move on to filtering with WHERE, sorting with ORDER BY, working with text, date, and number functions, aggregating data with GROUP BY and HAVING, writing subqueries, combining tables with JOIN, handling transactions, and finally creating tables with primary and foreign keys. Each step builds naturally on the last — and a set of real-world practice exercises at the end ties everything together. LEARN BY DOING, NOT JUST READING Every lesson pairs plain-English explanations with real query examples and hands-on fill-in-the-blank exercises. The sample data is drawn from everyday scenarios — a zoo, restaurant reservations, an online store — so you always know why a query matters, not just how to type it. Answer a question and you'll see instantly whether you got it right, so nothing slips through the cracks. BUILD A HABIT THAT STICKS A progress bar shows how far you've come, and a daily streak counter keeps you coming back. Your progress is saved right on your device, so you can always pick up exactly where you left off. MADE FOR YOU IF YOU'RE… • Brand new to SQL and looking for a friendly place to start • Taking your first steps toward data analysis or engineering • Looking to learn in the small pockets of time throughout your day LITTLE THINGS YOU'LL LOVE • No sign-up required — just open it and start learning • Works offline, wherever you are • An AI helper on hand whenever a question comes up • Clean syntax highlighting and a simple, distraction-free design Start today and learn at your own pace. Let Us SQL makes mastering data feel easy.