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-- Or maybe conf_size_counts ??
...
  AND c1.parent_client_id = c2.id
  AND exp.extension_id = ext.id
GROUP BY c2.id;

SELECT c.id, c.name, cs.conf10, cs.conf20, cs.conf30
FROM client c LEFT OUTER JOIN conf_sizes cs ON c.id = cs.id
WHERE c.level = 50
  AND c.status = 1