SQL Mental Model & Query Execution Order
Logical query processing order, JOIN semantics and row-count reasoning, set operations, three-valued NULL logic, and tracing a query step by step
The foundational SQL subject every data engineering interview loop assumes you already have: the logical order the database actually evaluates a query in (FROM/JOIN, WHERE, GROUP BY, HAVING, SELECT, DISTINCT, ORDER BY, LIMIT) versus the order you type it in, why that gap explains half of the 'why doesn't this query work' confusions candidates hit, JOIN types reasoned about by row count rather than memorized as Venn diagrams, UNION/INTERSECT/EXCEPT set semantics, the three-valued logic that makes NULL comparisons and the NOT IN trap so dangerous, GROUP BY/HAVING as a mental model rather than a syntax rule, and a fully worked step-by-step trace of a non-trivial query.