Given a schema and a question, pick the correct query. JOINs, window functions, aggregation, and the NULL traps that break naive answers.
Each drill gives you the DDL, the question, and four plausible queries. Usually only one handles NULLs, ties, or duplicate rows correctly, and the explanation walks through the rows where the others diverge. Cards are scheduled with spaced repetition: missed cards return sooner, and known cards space out.