Three-valued logic (NOT IN)

SQL uses three-valued logic (TRUE, FALSE, UNKNOWN) instead of classical boolean logic. This fundamentally changes how compound conditions behave and breaks some intuitive rules. Three-Valued Logic Boolean operations in SQL don't just have TRUE and FALSE outcomes; UNKNOWN is a third possibility that propagates through logical operators. Truth Tables with UNKNOWN Common Pitfalls Three-valued logic creates counterintuitive behaviors that cause subtle bugs even for experienced developers. The NOT IN Trap De Morgan's Laws Break Down The Excluded Middle