If I may,
 
I read Bruce's response to Tony's response to Jerry's original post.  I also
talked with Jerry about his post.
 
What Jerry was trying to get at,  and I am paraphrasing here, is that in his
application, which uses reals, the diddle the sign bit of the representation,
whilst leaving the actual value as 0.  This effectively makes it a negative
zero, which is an oximoron.  It is a valid way to denote a null item,
provided that the software knows to look to the sign bit.  There is
definitely a difference between null item and zero value.  IMAGE/SQL does not
provide for this facility, but most if not all RDBMSes do.  It is in actual
fact, part of the SQL92 standard.
 
This little trick will work only for reals, be they HP3000 or IEEE reals.  If
there is no value in the exponent part, and in the mantissa, the sign can and
in Jerry's case, is used to denote null item.  But all software must know
about this or it will not work.
 
This trick will not work with a 2s-complement value, because as soon as you
touch the zeroth bit, the representation has a value not equal to 0.  The
value depends on whether it is a 16-bit: -32,768 or 32-bit -2,147,483,648.
 Touch a bit and you get a non-zero value.  For a logical value, the zeroth
bit represents 32,768 and 2,147,483,648.
 
For ASCII, definitely this trick will work, simply put in binary zeroes. But
do you need to put in binary 0s for each character, or just the first one.
 What happens if there are real characters after the first position.  What is
binary zero is a valid representation for an application.  Binary 0s do not a
null item make.
 
However, in the grand scheme of things, bottom line, end of the day, when all
is said and done, etc... It is still a kludge.  There should be a way whereby
IMAGE/SQL can store and indicate when requested, the fact that a field is a
null item.  ALLBASE does it, all RDBMSes do it.  That was, I believe, the
intent of the original post: Show how one site had gotten around a limitation
with a kludge, and show the need for this ability to store and indicate null
items.
 
Sorry for the interruption.
 
Kind regards,
 
Denys. . .