# SQLDOU — SQL Query Execution Visualizer > SQL Query Execution Visualizer (iPhone/iPad app by Fuji Bit Inc..) - Source: https://appshunter.io/ios/app/sqldou/id6760616192 (this page in markdown: same URL + `.md`) - Developer: [Fuji Bit Inc.](https://appshunter.io/developer/1874205957) - Category: Education, Utilities - Price: Free with in-app purchases - Age rating: 4+ - Requires: iOS 16.0 · 10 MB - Languages: English, French, German, Japanese, Korean, Portuguese, Chinese, Spanish, Chinese - Released: 2026-03-22 - Data updated: 2026-06-30 - User reviews in markdown: https://appshunter.io/ios/app/sqldou/id6760616192/reviews.md ## What is SQLDOU? 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 ## Pricing and in-app purchases Base price: Free. **One-time purchases** | Purchase | Price | Type | | --- | --- | --- | | SQLDOU Pro | $5.99 | one-time | ## Version history (last 5 releases) ### 4.0 — 2026-06-13 Custom Quiz Mode now supports multiple tables and JOIN. The app is now available in 9 languages. - Custom Quiz Mode: define multiple tables and build JOIN queries with the visual builder - 9-language support: English, Japanese, Simplified Chinese, Traditional Chinese, Spanish, Portuguese (Brazil), Korean, German, French - UI polish and bug fixes throughout the app ### 3.0 — 2026-05-03 Custom Quiz Mode added (Pro only). Android version fully rewritten for full feature parity with iOS. - Custom Quiz Mode -- define your own table, columns, rows and SQL, then save and run - Per-column type (INT/TEXT/FLOAT/DATE/BOOL), primary key selection, index choice (B-Tree/Hash) - Visual SQL builder (assemble SELECT/WHERE/ORDER BY via pickers, SQL is auto-generated) - Go beyond the built-in missions and reproduce real-world scenarios you care about - Save multiple quizzes; edit, run or delete any time - Always-visible copyright on Home screen - Android rebuilt to full iOS parity (3 modes / 20 missions / prediction quizzes / Pro diagnostics / NodeConfigSheet / MissionResultOverlay / EventLog / ScanNarration) - UI polish and bug fixes throughout the app ### 2.1 — 2026-03-28 We have fixed the bug. ### 2.0 — 2026-03-24 Now available in English! Full bilingual support (English / Japanese). • English localization -- all UI, missions, quizzes, diagnostics, and logs • Improved splash screen animation • Bug fixes and performance improvements ### 1.0 — 2026-03-23 No release notes. ## More apps by Fuji Bit Inc. - [SUUGAKU](https://appshunter.io/ios/app/suugaku/id6759102221) - [BUTURAKU](https://appshunter.io/ios/app/buturaku/id6759213255) - [PAKEDOU](https://appshunter.io/ios/app/pakedou/id6759820254) - [OKURERUNA！](https://appshunter.io/ios/app/okureruna/id6759956971) - [HARAKU](https://appshunter.io/ios/app/haraku/id6759958253) - [KORAKU](https://appshunter.io/ios/app/koraku/id6760215571) All apps by Fuji Bit Inc.: https://appshunter.io/developer/1874205957 --- *Data collected daily from the US App Store and indexed by [AppsHunter](https://appshunter.io/). User reviews are verbatim App Store reviews. Ratings, prices and chart positions refresh continuously; this snapshot is from 2026-06-30.*