Teaches SQL concepts and query reasoning for MySQL, PostgreSQL, and SQLite in Japanese.
npx skills add https://github.com/kohdice/dotfiles --skill sql-tutor-kohdice
Please help me install this Agent Skill. Skill: sql-tutor Source: https://github.com/kohdice/dotfiles/tree/main/config/agents/skills/sql-tutor Command: npx skills add https://github.com/kohdice/dotfiles --skill sql-tutor-kohdice
NOT IN with NULL or WHERE after LEFT JOIN return incorrect rows. - Use Case: A beginner pastes a LEFT JOIN query that unexpectedly drops rows and asks why; the tutor explains that the WHERE clause on the right table turned the join into an inner join, shows the fixed query, and teaches the habit of checking row counts clause by clause. ## Quick Start Ask the tutor to explain why your SQL query returns unexpected rows, in Japanese, mentioning which database engine you use.High-intent search queries and answers about installing and using this skill.
This tutor explains INNER JOIN and LEFT JOIN using minimal tables with sample rows, showing which rows pair and which survive. It explains the ON condition as deciding which rows combine, with step-by-step intermediate result diagrams.
A WHERE condition on the right table after a LEFT JOIN silently turns it into an inner join, because NULL values fail the comparison. Move the condition into the ON clause so it decides which rows pair instead of which rows survive.
No, comparing with = NULL is never true because NULL means unknown, not empty. Use IS NULL or IS NOT NULL, and COALESCE to define what should happen to unknown values explicitly.
It covers MySQL, PostgreSQL, and SQLite. The engine is resolved from what the user names, project evidence like DATABASE_URL or migration files, or portable standard SQL when neither is available.
No, live database execution is explicitly out of scope, as are application implementation, refactoring, and code review tasks. It focuses on teaching query logic with self-contained example tables you can paste into any client.
PostgreSQL and MySQL with ONLY_FULL_GROUP_BY reject non-aggregated columns because the value's source row is unspecified. SQLite accepts it and picks an arbitrary row, so the strictest engine is treated as the teacher.
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