Two SQL NULL Traps: Missing JOIN Rows and UNIQUE Constraints That Do Nothing

Originally published at recca0120.github.io JOIN two tables on a nullable column and the rows disappear. Insert two NULLs into a UNIQUE column and neither insert fails. Both problems share the same root: SQL defines NULL as unknown, and unknown is never equal to anything — including another unknown. What NULL Actually Means In SQL, NULL means "unknown value" — not zero, not empty string, not false. SELECT NULL = NULL ; -- Result: NULL, not TRUE SELECT NULL IS NULL ; -- Result: TRUE NULL = NULL r