Thanks for the further input from Milan and Adam.



I have some further questions based on Milan's advice since I have not come
across the concept of NULL in Boolean

evaluation before.



If NULL=NOTNULL evaluates to NULL does

NULL<>NOTNULL also evaluate to NULL?



Does NULL within any Boolean expression such as

IF ((Null OR False) and True) make the whole expression evaluate to NULL?



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



Graeme





From: firebird-support@yahoogroups.com
[mailto:firebird-support@yahoogroups.com] On Behalf Of Adam
Sent: Thursday, 16 February 2006 10:15 AM
To: firebird-support@yahoogroups.com
Subject: [firebird-support] Re: The New command in triggers



--- In firebird-support@yahoogroups.com, Christian Danner
<christian@...> wrote:

>
> So if the number of comparisons really matters (3:5):
>
> if (not( ( (old.field is null)
> and (new.field is null))
> or (old.field = new.field)))
> then
> ...
>
> Christian
>

Milan was right, I forgot about the null OLD=NEW resolving to NULL in
some cases. Of course if you use his selection you must make sure
CompleteBooleanEvaluation = 0 in firebird.conf (this is the default btw).

Personally I think this probably needs some attention. It is a common
problem and given triggers are not supported on a field by field
level, perhaps a function like

FieldChanged(FIELD) could return true if

a) NULL state was different between OLD and NEW; or
b) Value was different if both not null

So then the logic would be:

IF FIELDCHANGED(MYFIELD) THEN ....
ELSE ....

Adam





++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Visit http://firebird.sourceforge.net and click the Resources item
on the main (top) menu. Try Knowledgebase and FAQ links !

Also search the knowledgebases at http://www.ibphoenix.com

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++





SPONSORED LINKS


Technical
<http://groups.yahoo.com/gads?t=ms&k=Technical+support&w1=Technical+support&
w2=Computer+technical+support&w3=Compaq+computer+technical+support&w4=Compaq
+technical+support&w5=Hewlett+packard+technical+support&w6=Microsoft+technic
al+support&c=6&s=196&.sig=-XIO8GxY6hqd3NaD5WSEyw> support

Computer
<http://groups.yahoo.com/gads?t=ms&k=Computer+technical+support&w1=Technical
+support&w2=Computer+technical+support&w3=Compaq+computer+technical+support&
w4=Compaq+technical+support&w5=Hewlett+packard+technical+support&w6=Microsof
t+technical+support&c=6&s=196&.sig=B29J78SYXnNTjjMFBMznqA> technical
support

Compaq
<http://groups.yahoo.com/gads?t=ms&k=Compaq+computer+technical+support&w1=Te
chnical+support&w2=Computer+technical+support&w3=Compaq+computer+technical+s
upport&w4=Compaq+technical+support&w5=Hewlett+packard+technical+support&w6=M
icrosoft+technical+support&c=6&s=196&.sig=7_je1A94xs82CFXUjEqA6g> computer
technical support


Compaq
<http://groups.yahoo.com/gads?t=ms&k=Compaq+technical+support&w1=Technical+s
upport&w2=Computer+technical+support&w3=Compaq+computer+technical+support&w4
=Compaq+technical+support&w5=Hewlett+packard+technical+support&w6=Microsoft+
technical+support&c=6&s=196&.sig=2zMAuRCo5cJrVBr1Bxa3_w> technical support

Hewlett
<http://groups.yahoo.com/gads?t=ms&k=Hewlett+packard+technical+support&w1=Te
chnical+support&w2=Computer+technical+support&w3=Compaq+computer+technical+s
upport&w4=Compaq+technical+support&w5=Hewlett+packard+technical+support&w6=M
icrosoft+technical+support&c=6&s=196&.sig=_ytYU7aXb57AVaeUfmvLcA> packard
technical support

Microsoft
<http://groups.yahoo.com/gads?t=ms&k=Microsoft+technical+support&w1=Technica
l+support&w2=Computer+technical+support&w3=Compaq+computer+technical+support
&w4=Compaq+technical+support&w5=Hewlett+packard+technical+support&w6=Microso
ft+technical+support&c=6&s=196&.sig=4hRo6NXYavRAbTkaYec5Lw> technical
support



_____

YAHOO! GROUPS LINKS



* Visit your group "firebird-support
<http://groups.yahoo.com/group/firebird-support> " on the web.

* To unsubscribe from this group, send an email to:
firebird-support-unsubscribe@yahoogroups.com
<mailto:firebird-support-unsubscribe@yahoogroups.com?subject=Unsubscribe>

* Your use of Yahoo! Groups is subject to the Yahoo!
<http://docs.yahoo.com/info/terms/> Terms of Service.



_____



[Non-text portions of this message have been removed]