sql
SQL — engine-agnostic query craft
This skill is the portable query-writing layer that sits above any one database engine. It owns
the SELECT-side craft: joins and what each does to row count and NULLs, window functions
(PARTITION/ORDER/frame), CTEs (including recursive), aggregation (GROUP BY/GROUPING SETS/HAVING),
set operations (UNION/INTERSECT/EXCEPT), conditional logic (CASE/COALESCE/NULLIF), and the
NULL three-valued-logic traps that quietly corrupt results across every engine. You write queries a
reviewer accepts on Postgres, MySQL 8, SQLite, DuckDB, SQL Server, or BigQuery with minimal change, and
you flag exactly where a construct is non-portable and what the dialect substitute is. The target
standard is SQL:2023 (ISO/IEC 9075:2023), the ninth edition published June 2023; window functions
have been standard since SQL:2003, so they are safe to assume everywhere.
This is about thinking in sets and frames, not about one product's planner, DDL, indexing, or ops.