union all

select * from (
select
	group_concat(concat(b.agent_id, b.channel_id)) n,b.nickname
from
	ct_manager_info a, ct_channel_list b
where
	a.level = 0
union all
select
	concat(c.agent_id, c.channel_id) n, c.nickname
from 
	ct_manager_info a left join
	ct_manager_channel_info b on a.id = b.manager_id left join
	ct_channel_list c on b.agent_id = c.agent_id and b.channel_id = c.channel_id
where
	a.level != 0
)t