SQL Query Foundations

Description

Write correct, readable SQL from scratch - SELECT and projection, WHERE filtering (comparisons, BETWEEN, IN, LIKE, IS NULL), NULL semantics, ORDER BY, GROUP BY and HAVING with aggregates, every join type and when each is correct, subqueries (scalar, IN, EXISTS), CTEs, UNION/INTERSECT/EXCEPT, OFFSET vs keyset pagination, and INSERT/UPDATE/DELETE with RETURNING. Use whenever the user is learning SQL or writing first queries, asks what a JOIN does, hits a GROUP BY error, combines result sets, is confused by NULL, paginates results, or gets unexpected, missing or duplicated rows.