From: "Frank Bax"
> At 11:07 AM 4/27/05, Jigal van Hemert wrote:
> >So, if we would define that the key entry "0-NULL-Whatever" equals
> >"0-NULL-Whatever" (which MySQL is capable of if you look at the
> >of UNIQUE indexes with BDB tables) then allowing NULLs as part of a key
> >entry would not permit duplicate entries. At least not more than allowing
> >other values.
> You cannot "define" that those keys are equal, because SQL standard states
> that "0-NULL-Whatever" is *always* *not-equal* to "0-NULL-Whatever".
Allowing only a single NULL entry in a UNIQUE index of BDB tables would also
imply that in at least one case a new NULL "value" is treated as equal to
the NULL that is already present in the index. A "duplicate key" error would
IMHO mean that a value that one tried to insert is equal to a value that is
already present in the index...
But you are also not quite right stating that "0-NULL-Whatever" is not equal
to "0-NULL-Whatever". Comparing two NULLs will not result in equal or not
equal, but in unknown (represented by NULL) ;-P