SQL aggregation semantics
Review SQL NULL comparisons, JOINs, and COALESCE
Review synthetic NULL, inner/left JOIN, and COALESCE cases, then save a local aggregation note.
Review a SQL semantics case
Choose a synthetic table case to review.
No SQL is executed. No database, upload, request, or storage is used.
Before aggregation
Agree whether missing values are filtered, unmatched rows are preserved, or a display fallback is used; a fallback does not restore source data.