Fastest way to find parents with only one child?

Question

I have two very large tables (both >6 million rows) in an oracle 8i DB. They have a parent child relationship and I would like to construct a query to give me the parents that have only one child......syntactically, what's the best way to construct this query?
I was going to try:
select join_id
FROM parent
where join_id in (select join_id, count(join_id)
FROM child
group by join_id
having count(*)=1)
but then I realized that the subselect has two columns to return and the primary query is only expecting one column, so this will not work.
I suspect there's a quick and dirty way to find out what parents have only one child....
I thought of using rowid's but am not sure this is the best way and in the example below I tried, I'm having problems b/c of functions not being allowed in the where clause.....
select join_id
from child d
where rowid in (select min(rowid)
          FROM child s
          WHERE min(d.rowid)=max(s.rowid)
          AND s.join_id=d.join_id))
Any thoughts?

Answer 1

The two tables are order_header and order_detail. The order_header carries order specific information and the detail contains item specific information.
So if you had ordered three separate products, you would have:
one row in the order_header table (parent)
and three rows in the order_detail table (child)
They are linked by order_number.
I presented the problem this way to make it more accessible to more posters.....;)
One possible solution that I've thought of for my problem is this:
select join_id
from child_table d
where (d.rowid, d.rowid) IN (select min(rowid) MIN_ROW
               ,max(rowid) MAX_ROW
               FROM child_table
               WHERE s.str_order_number=d.str_order_number
               AND s.date>='30-JAN-2005'
               AND s.date<='31-JAN-2005'))
I think this might work because I think that we can safely assume that if the minimum rowid and the maximum rowid (with the same join_id ) are the same then there is only one child record.....is this logic correct?