mysql default value select code example

Example: 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