Select rows which have no match in other table?

I have this structure
tbl_site_urls: storing urls of a site
tbl_camp_urls: storing urls of a campaign, after handling these urls
tbl_site_urls
id - site_id - url
1 - 181 - url1
2 - 182 - url2
3 - 189 - url3

tbl_camp_urls
id - camp_id - site_id - url
1 - 5 - 181 - url1


if i want to list the urls need to be handle

select url from tbl_site_urls
join site on tbl_site_urls.site_id = site.id AND site.valid=1

then in a loop operation i insert the handled urls into the tbl_camp_urls

I want to list the urls that which have not handled yet.
it is like a comparison operation.


The results I'm expecting is
2 - 182 - url2
3 - 189 - url3

there are 2 ways to do this. you should experiment to see which is more efficient with your tables (usually it's the 2nd one)

1.

2.