> The same as before. s is A, t is B, but s+t is A, when A is not NULL and

s+t is

> B, when A is NULL.

Change to:

var s,t:String;

qry.edit;

if qry.fieldbyname('A').isnull then s:=''
else s:=qry.fieldbyname('A').asString;

if qry.fieldbyname('B').isnull then t:=''
else t:=qry.fieldbyname('B').asString;

qry.fieldbyname('A').asString:=s+t;

qry.post;


Or,
If the result of s+t is empty and you want use Null instead of a empty
string, you can use:
....
....
if (s+t)='' then
qry.fieldbyname('A').Clear
else qry.fieldbyname('A').asString:=s+t;
qry.post;


PS: Maybe there is a small bug in IBO in this case, when the first value is
null.
qry.fieldbyname('A').asString:=qry.fieldbyname('A').asString+qry.fieldbyname
('B').asString;
should give the correct result in my opinion.

Luiz.