mysql stored function simple login, extract int from set - mysql

Question

this is my first mySQL stored-function 'cause i've always managed such things with php.
I want a function that checks if a user can log in my client-area.
I wrote the code above:
DELIMITER $$
CREATE FUNCTION `A05`.`login` (user VARCHAR(64),pass VARCHAR(64)) RETURNS INT
BEGIN
declare num_rows int;
declare id int;
SELECT (#num_rows:=COUNT(*)),(#id:=`Credential_id`) FROM `A05`.`Credentials` where `Credential_UserName` = user;
if num_rows = 0 then
-- user not present
return (-1);
end if;
-- user present, checking password
SELECT (#num_rows:=COUNT(*)),(#id:=`Credential_id`) FROM `A05`.`Credentials` where `Credential_id` = id AND `Credential_PASSWORD` = SHA1(pass);
if num_rows = 0 then
-- user presente, password incorrect
INSERT INTO `a05`.`Events` (
`Event_id` ,
`Event_RegistrationDate` ,
`Event_VariationDate` ,
`Event_State`,
`Event_Notes`,
`Event_SenderId`,
`Event_Type`
)
VALUES (
NULL , NOW(), NOW(), 'wp', NULL, id, 1
);
return (-2);
end if;
-- user present, password correct
UPDATE `A05`.`Credentials` SET `Credential_LastAccess`=NOW() where `Credential_id` = id;
INSERT INTO `a05`.`Events` (
`Event_id` ,
`Event_RegistrationDate` ,
`Event_VariationDate` ,
`Event_State`,
`Event_Notes`,
`Event_SenderId`,
`Event_Type`
)
VALUES (
NULL , NOW(), NOW(), 'ok', NULL, id, 0
);
return id;
END
I think that the syntax should be right except for the last statement return id cause i return a set instead of an integer.
The problem is that when i try to store this function on mysql i get this error:
Error 1415: Not allowed to return a result set from a function
Then i changed the last statement from 'return id' to 'return 0' for testing purpose but i keep getting the same error.
Probably is a newbie error cause it's the very first time for me on sql "advanced" scripting.
Thank you very much in advance!