CREATE TABLE TEST (
    ID   INTEGER NOT NULL,
    VAL  INTEGER NOT NULL
);

execute block
as
begin
  insert into test (id, val) values (1, 100);
  insert into test (id, val) values (2, (select val from test where id = 1));
end

execute block
as
declare v integer;
begin
  insert into test (id, val) values (1, 100);
  select val from test where id = 1 into :v;
  insert into test (id, val) values (2, :v);
end