Hi,
On 1-10-2011 21:51, Halász Sándor wrote:
It is, of course, generally considered more natural to make equality
primary, not inequality, but that symbol that MySQL uses for
NULL-safe equality,<=>, looks much more like inequality than
equality.
The whole concept and the name of this operator is wrong IMO. There is
nothing NULL-*safe* about it. Equal and unequal operators are in fact
more NULL-*safe* than <=>.
But if I write IF A <> B THEN often I want it NULL-safe, for if
one is NULL and the other not, I want that true
You are not using NULL as the original concept of it was. NULL means
that the value is undefined or unknown.
If a value is undefined it may have *any* value.
So, if you evaluate (A = NULL) the NULL part can have *any* value, even
A. The result of this compare can only be NULL, because it is not known
whether it's equal or unequal.
Because of this (NULL = NULL) must be NULL too. (NULL <> NULL) must also
result in NULL. The result is just as undefined/unknown as both values
which were compared.
The usual solution in the case you describe is that you use a normal
value in the range of the field type which is not used normally. E.g.
for an INT field where you only use values of zero or larger you can use
e.g. -1 as a special value.
If you insist on using NULL and the crazy <=> operator you can use NOT
to invert it:
SELECT NOT(A <=> B);
--
Kind regards / met vriendelijke groet,
Jigal van Hemert.
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