select count(distinct hostname) as unique_hostnames, 
       count(*) as record_count
from my_table 
where clean_hostname is null

##</code><code>##

select hostname, count(*) cnt
from the_table 
where clean_hostname is null
group by hostname
order by count(*) desc

##</code><code>##

update my_table set clean_hostname = hostname, confidence=100.0
where hostname in ('first.correct.host', 'second.correct.host', <etc>)
or hostname like '%.mybiz.com'
or hostname like '%.joescompany.net'

##</code><code>##

update my_table set clean_hostname = '*GARBAGE*', confidence=100.0
where hostname in ('first.correct.host', 'second.correct.host', <etc>)
or hostname like '%.spamzalot.foo'

##</code><code>##

update my_table set clean_hostname = hostname||'houston.com', confidence=95.0
where hostname like '%.q9ix'