gistfile1.sql

-- Model at http://i.imgur.com/kuxy2.png
-- Find all channels that have tags 'tag 1' and 'tag 2'

select c.channel
from channels c
where c.id in (select ct.channelid
               from channel_tags ct 
               join tags t on ct.tagid = t.id
               where t.tag = 'tag 1')
  and c.id in (select ct.channelid
               from channel_tags ct 
               join tags t on ct.tagid = t.id
               where t.tag = 'tag 2')

select c.channel
  from channels c
  join channel_tags ct on c.id = ct.channelid
  join tags t on t.id = ct.tagid
 where t.tag in ('tag 1', 'tag 2')
group by c.channel, 
         ct.channelid
  having count(ct.channelid) = 2

select c.channel
from channels c, channel_tags ct, tags t
where ct.tagid = t.id and ct.channelid = c.id
and (t.tag in ('tag 1', 'tag_2'))
group by c.id
having count(c.id) = 2