NULLs and Three-Valued Logic

NULL means unknown, so SQL relies on three-valued logic where comparisons with NULL return UNKNOWN, not TRUE or FALSE. This is the quiet source of wrong results: = NULL never matches, NOT IN quietly drops every row when the subquery contains a NULL, and aggregates and outer joins treat NULL in surprising ways. AI, ML, and GenAI interviews probe it because confidently wrong queries that pass review are worse than queries that error, and NULL handling is where they hide.