In a stored procedure, a variable has been assigned a value.
Later on, when it is used to receive select result which does not
return any result, isn't the variable suppossed to be null now?

However, it is not. Please see a simple SP below. When the SP is
executed, the expected result should ID1 has a value and ID2 should be
NULL. However, the ID1 and ID2 both get the same value.

Is it by designed? I know, the workaround is simple, but it makes us
easily slipped forgetting to reset the variable to null before
executing a select statement.

CREATE PROCEDURE DOES_NOT_WANT_TO_BE_NULL
RETURNS (
ID1 INTEGER,
ID2 INTEGER)
AS
declare variable ID_RESULT integer;
BEGIN
--ID_RESULT will get some value
select RDB$RELATION_ID from RDB$DATABASE into ID_RESULT;
ID1 = ID_RESULT;

--Now ID_RESULT should be NULL, but it retains its old value
select RDB$RELATION_ID from RDB$DATABASE
where 'A dog talks' = 'Impossible' into ID_RESULT;
ID2 = ID_RESULT;
SUSPEND;
END


--
Bambang P.