Graeme,

>Does NULL within any Boolean expression such as
>
>IF ((Null OR False) and True) make the whole expression evaluate to NULL?

only if it makes sense (as in your example). 'if (true or null)' on
the other hand returns 'true'.

>Is it valid to test the result of a Boolean expression for NULL as well as
>true and false?
>
>Eg.
>
>If (New.ID=Old.ID) is Null then .. Whatever

AFAIK such an expression is not allowed (yet). (New.ID=Old.ID) is
'untyped'.

Regards,

Christian