Select inside function not working as expected - function

Question

I have created a function to encapsulate a select on PL/SQL.
But the result I get from using the function is different that the one I get from using the select on it's own.
CREATE OR REPLACE FUNCTION fecha_ultimo_trasiego(fermentacion IN VARCHAR2)
RETURN DATE
IS fecha DATE;
BEGIN
SELECT (
SELECT fecha
FROM (
SELECT t.id, t.fecha
FROM TrasiegoAux t
WHERE t.fermentacion=fermentacion
ORDER BY t.fecha desc
)
WHERE ROWNUM=1
) INTO fecha FROM DUAL;
RETURN(fecha);
END;
Is there something wrong on the declaration of my function? Right now it always returns the same value no matter what parameter it gets as input.
The expected output is the date of the last Trasiego (the max date of the table).
This is my structure:
CREATE TABLE Fermentacion(
id VARCHAR2(36)
);
CREATE TABLE Trasiego(
id VARCHAR2(36),
fecha DATE not null,
fermentacion VARCHAR2(36) REFERENCES Fermentacion,
temperatura NUMBER(8) not null,
litrosFinal NUMBER(8) not null,
PRIMARY KEY(id, fermentacion)
);
create or replace view TrasiegoAux as select id, fecha, fermentacion from Trasiego;
Here is some data:
Insert into FERMENTACION (ID) values ('4');
Insert into FERMENTACION (ID) values ('9');
Insert into TRASIEGO (ID,FERMENTACION,TEMPERATURA,LITROSFINAL,FECHA) values ('1','4',20,190,to_date('04-OCT-16','DD-MON-RR'));
Insert into TRASIEGO (ID,FERMENTACION,TEMPERATURA,LITROSFINAL,EMPLEADOLABORATORIO,FECHA) values ('2','4',20,180,to_date('11-OCT-16','DD-MON-RR'));
Insert into TRASIEGO (ID,FERMENTACION,TEMPERATURA,LITROSFINAL,FECHA) values ('3','9',20,190,to_date('04-OCT-16','DD-MON-RR'));
Given that data I would expect that this:
DECLARE
fecha date;
BEGIN
fecha :=fecha_ultimo_trasiego(4);
DBMS_OUTPUT.PUT_LINE(fecha); //'11-OCT-16'
fecha :=fecha_ultimo_trasiego(4);
DBMS_OUTPUT.PUT_LINE(fecha); //'04-OCT-16'
END;

Answer 1

It's most likely to be because you're passing in the parameter with the same name as the column. That's not a good idea, as it causes all sorts of strangeness.
You can either amend the parameter name, e.g.:
CREATE OR REPLACE FUNCTION fecha_ultimo_trasiego(p_fermentacion IN VARCHAR2)
...
WHERE t.fermentacion = p_fermentacion
...
or you could qualify the parameter name in the query:
CREATE OR REPLACE FUNCTION fecha_ultimo_trasiego(fermentacion IN VARCHAR2)
...
WHERE t.fermentacion = fecha_ultimo_trasiego.fermentacion
...

Answer 2

You need to give the parameter a different name to any of the columns in TrasiegoAux, or else prefix it with the function name, e.g.
WHERE t.fermentacion = fecha_ultimo_trasiego.fermentacion
or else (after renaming the parameter):
WHERE t.fermentacion = p_fermentacion
Edit: Boneist beat me to it.
You could probably simplify the code a bit, though, as the select ... from dual wrapper doesn't seem to be needed:
create or replace function fecha_ultimo_trasiego
( p_fermentacion in trasiegoaux.fermentacion%type )
return date
as
l_fecha date;
begin
select fecha into l_fecha
from ( select t.fecha
from trasiegoaux t
where t.fermentacion = p_fermentacion
order by t.fecha desc )
where rownum = 1;
return l_fecha;
end;