select a.field1, a.field2, a.field3,
         (
            select b.field1, b.field2, b.field3 
            from table2 b
            where b.id=a.id and ...      
         )
from table1 a

execute block
returns (text varchar(20), c int) as
begin
  text = null; c = null;
  select 'Text 1', 0 from rdb$database where false
  into text, c;
  suspend; -- Переменные не заполнены

  text = null; c = null;
  select 'Text 2', count(*) from rdb$database where false
  into text, c;
  suspend; -- А тут все в порядке
end

for select ....
into :V1..:Vn 

SELECT ... INTO :VAR1 DEFAULT NULL, :VAR2 DEFAULT NULL