Update your stored function as follows
DELIMITER $$
DROP FUNCTION IF EXISTS `check2_login`$$
CREATE FUNCTION `check2_login`(p_username VARCHAR(30),p_password VARCHAR(30),role VARCHAR(20)) RETURNS BOOL
BEGIN
DECLARE retval INT;
IF role = "customer" THEN
SELECT COUNT(custid) INTO retval FROM customer WHERE custid = p_username and pwd = p_password;
IF retval != 0 THEN
RETURN TRUE;
ELSE
RETURN FALSE;
END IF;
ELSEIF role = "executive" THEN
SELECT COUNT(execid) INTO retval FROM executive WHERE execid = p_username and pwd = p_password;
IF retval != 0 THEN
RETURN TRUE;
ELSE
RETURN FALSE;
END IF;
ELSEIF role = "admin" THEN
SELECT COUNT(empid) INTO retval FROM employee WHERE empid = p_username and pwd = p_password;
IF retval != 0 THEN
RETURN TRUE;
ELSE
RETURN FALSE;
END IF;
ELSE
RETURN FALSE;
END IF;
END$$
DELIMITER ;
hope this will help you...!