Python slang

[Bernd Nawothnig]

On 2016-08-06, Chris Angelico wrote:
> On Sun, Aug 7, 2016 at 5:37 AM, Bernd Nawothnig
><Bernd.Nawothnig at t-online.de> wrote:
>>> But SQL's NULL is a cross between C's NULL, IEEE's NaN, Cthulhu, and
>>> Emrakul.
>>
>> SQL NULL has the semantic of "unknown". So if one or both operands of
>> a comparison (or any other operation) are unknown the result is
>> unknown too. And that means NULL.
>
> That's not entirely accurate, and it doesn't explain why NULL
> sometimes behaves as if it's a genuine value, and sometimes as if it's
> completely not there. For instance, taking the average of a column of
> values ignores NULLs completely, and COUNT(column) is the same as
> COUNT(column) WHERE column IS NOT NULL; but in some situations it
> behaves more like NaN:
>
> rosuav=> select null or true, null or false, null and true, null and false;
>  ?column? | ?column? | ?column? | ?column?
> ----------+----------+----------+----------
>  t        | NULL     | NULL     | f
> (1 row)
>
> Anything "or true" has to be true, so NULL OR TRUE is true. And then
> there are the times when NULL acts like a completely special value,
> for instance in a foreign key - it means "there isn't anything on the
> other end of this relationship", and is perfectly legal. Or in a
> SELECT DISTINCT, where NULL behaves just like any other value - if
> there are any NULL values in the column, you get back exactly one NULL
> in the result.

Thanks for that additions and corrections.




Bernd

-- 
no time toulouse