provide hardcoded value in the mysql query
SELECT "" as Serial,
"" as Object_ID,
"" as ObjectDEsc,
"" as Object_Num,
"--(UNKNOWN -- )" as DropList
UNION
SELECT CAST(p.Serial AS UNSIGNED INTEGER) as Serial,
p.Object_ID,
p.Part_ID,
st.Description AS ObjectDesc,
s.Object_Num,
concat(Serial,' (',st.Desc,' #',s.Object_Num,')') as DropList
FROM Parts p LEFT JOIN Objects s ON p.Object_ID = s.Object_ID
LEFT JOIN ObjectTypes st ON s.ObjectType_ID = st.ObjectType_ID