Database Software

Hello!

I'm new in spatial oracle but I really want to learn.
I have a table calle RegionC where I have the coordenates of all the regions in Italy and each region has a number GRUPO. I divided Italy in 5 GRUPO (grups) so now I want the coordenates of each GRUPO with all the regions in it, I mean the coordenates of the hole GRUPO.
That's why I used SDO_UNION, but I gives me nothing!!!
What am I doing wrong?
Please help me!!!!

SELECT SDO_GEOM.SDO_UNION(a.geom, m.diminfo, c.geom, m.diminfo)
FROM scott.regionc a, scott.regionc c, user_sdo_geom_metadata m
WHERE m.table_name = 'SCOTT.REGIONC' AND m.column_name = 'GEOM'
AND a.grupo = 3;

SELECT SDO_GEOM.SDO_UNION(a.geom, c.geom, 0.5)
FROM scott.regionc a, scott.regionc c, user_sdo_geom_metadata m
WHERE m.table_name = 'SCOTT.REGIONC' AND m.column_name = 'GEOM'
AND a.grupo = 3;

Thank you very much in advance!!!