CREATE GLOBAL TEMPORARY TABLE TMP (
    FLD1  INTEGER,
    FLD2  INTEGER
) ON COMMIT PRESERVE ROWS;

SET TERM ^ ;

CREATE OR ALTER PROCEDURE STRANGE_PROCEDURE
RETURNS (
    FLD1 INTEGER,
    FLD2 INTEGER)
AS
DECLARE VARIABLE Val INTEGER;
BEGIN
  DELETE FROM Tmp;

  INSERT INTO Tmp
  VALUES (1, 2);
  INSERT INTO Tmp
  VALUES (2, 3);
  INSERT INTO Tmp
  VALUES (NULL, 5);
  INSERT INTO Tmp
  VALUES (6, 7);

  --так работает
  UPDATE Tmp
  SET Fld2 = 10
  WHERE Fld1 = 2;

  --а так нет
  Val = (SELECT MAX(Fld2)
         FROM Tmp);

  UPDATE Tmp
  SET Fld2 = :Val
  WHERE Fld1 IS NULL;
  
  --и так тоже нет
  UPDATE Tmp
  SET Fld2 = (SELECT MAX(Fld2)
              FROM Tmp)
  WHERE Fld1 IS NULL;

  FOR SELECT Fld1, Fld2
      FROM Tmp
      INTO :Fld1, :Fld2
  DO
    SUSPEND;
END^