Hi I think this is a bug....

CREATE TABLE TableA (
Field1 VARCHAR(10)
);

CREATE TABLE TableB (
Field2 VARCHAR(10)
);

INSERT INTO TableA (Field1 ) VALUES ('A');
INSERT INTO TableA (Field1 ) VALUES ('B');
INSERT INTO TableA (Field1 ) VALUES ('C');

INSERT INTO TableB (Field2 ) VALUES ('B');
INSERT INTO TableB (Field2 ) VALUES ('C');
INSERT INTO TableB (Field2 ) VALUES ('D');


This work as expected
****************************************
Select TA.Field1, TB.Field2
From TableA TA
Full Join TableB TB On TA.Field1=TB.Field2
order by 1,2

Field1 Field2
========== ==========

A <null>
B B
C C
<null> D


This *DONT* work as expected
****************************************
Select TA.Field1, TB.Field2
From TableA TA
Full Join TableB TB On TA.Field1=TB.Field2
Where TB.Field2 Is Null



Field1 Field2
========== ==========

A <null>
B <null>
C <null>



Server Version WI-V1.5.0.4306 Firebird 1.5