Re: NULLs: theoretical problems?

Quoth V.J. Kumar:
> I do not understand why they are not equivalent.
>
> The first SQL predicate, according to your rules, evaluates to 'true':
> 'DEF t.a : (t.a = 5 OR TRUE)' -> 'FALSE OR TRUE' -> 'TRUE'
>
> The second SQL predicate also evaluates to 'true':
> 't.a = 5 OR TRUE' -> 'UNKNOWN OR TRUE' -> 'TRUE'
>
> What is the difference ?

If I understand it correctly, the first evaluates to TRUE for rows where t.a is defined, and FALSE for rows where t.a is not; whereas the second evaluates to TRUE regardless.

In other words, the DEF query is equivalent to

SELECT * FROM t WHERE t.a IS NOT NULL AND (t.a = 5 OR TRUE)

or, if we simplify, it's the difference between

SELECT * FROM t WHERE t.a IS NOT NULL -- the DEF query and
SELECT * FROM t -- The other one

But I believe these queries are equivalent:

SELECT * FROM t WHERE DEF t.a : t.a = 5
SELECT * FROM t WHERE t.a = 5

-- 
Jon