-- Or maybe conf_size_counts ??
CREATE VIEW conf_sizes AS
SELECT
  c2.id,
  sum(if(value<11,1,0)) AS "conf10",
  sum(if(value<20 and value>10,1,0)) AS "conf20",
  sum(if(value>20,1,0)) AS "conf30"
FROM extension ext, client c1, client c2, extension_prefs exp
WHERE ext.client_id = c1.id
  AND exp.param="conf_size"
  AND ext.type="conference"
  AND c1.parent_client_id = c2.id
  AND exp.extension_id = ext.id
GROUP BY c2.id;