I went on this old thread : http://www.georeference.org/forum/t105858.15#105925 and came with this:
SELECT [Surface_Sel], [CellCenter]
( SELECT [Surface].[Selection (I)] AS [Surface_Sel],
NewPoint([Surface].[Center X (I)],[Surface].[Center Y (I)]),
COORDSYS("Surface" AS COMPONENT) ) As [CellCenter]
FROM [Surface] where [Invisible (I)] = False)
SET [Surface_Sel] = True
It should be ok to select the wanted pixels.
I still need a second query to count the selected pixels.
Any better way ?