Exception message

Hi,
I create a function and I want for NO FOUND (no data found) or for too many rows to have some exceptions messages. If *** is null to have an exception message if *** return more than one row to have an exception message. How can I do this?
My function is :
CREATE FUNCTION test (
pid FLOAT
)
RETURNS VARCHAR(80)
BEGIN ATOMIC
DECLARE *** VARCHAR(80);
SET ***=( SELECT pname || ' ' || pname1 FROM test
WHERE pname5=pid);

RETURN ASSIGNMENTVARIABLE;

END