Variable in Select-Statement geht nicht (Stored Function)

[omata]

Vielleicht so...

SQL-Code:

CREATE FUNCTION `func_Mittelwert`(cnt INT) RETURNS DOUBLE
BEGIN
  DECLARE result DOUBLE;

  SELECT AVG(abzugskraft) INTO result
  FROM Messung1 x
  WHERE cnt > (SELECT COUNT(*)
               FROM Messung1 
               WHERE zeit < x.zeit);


  RETURN result;
END