3 value logic. Why is SQL so special?

SQL Apprentice Question
3 value logic. Why is SQL so special?

what would be the consequences of NULL=NULL being true?



Celko Answers

>>, what would be the consequences of NULL=NULL being true?


NULL=NULL should not be true.
NULL=NULL should not be false.
NULL=NULL should not be UNKNOWN.
NULL=NULL should be NULL. <<

NULL is a missing *attribute* value; UNKNOWN is a *logical* value. The
first rule of NULLs is that they propagate. You can easily set up
contradictions that depend on the order evaluation when you have a
BOOLEAN data type. All SQLK data types must allow NULLs by definition.


NULL OR TRUE = NULL -- by definition
UNKNOWN OR TRUE = TRUE -- by definition


NULL AND TRUE = NULL -- by definition
UNKNOWN AND TRUE = UNKNOWN -- by definition


This is why we have the IS [NOT] [TRUE | FALSE |UNKNOWN]
predicate in SQL-92