sqlite> select * from lookup;
node|id
1|1
1|2
2|4
2|6
sqlite> select * from tag;
tagid|data
1|bar
2|baz
3|geek
4|foo
5|bank
6|auto

select id from tag where not exists (select tagid from lookup);
# I am expecting the following result: 3, 5

select
    id
from
    tag t
where
    not exists (
        select
            'x'
        from
            lookup l
        where
            l.tagid = t.id -- correlation
    );

select
    t.id
from
    tag t
        left outer join
    lookup l
        on t.id = l.tagid
where
    l.tagid is null;

delete from
    tag
where
    not exists (
        select
            'x'
        from
            lookup l
        where
            l.tagid = tag.id
    );