unmatched

I seem to have a serious brain fart going here. I have two tables, and
I want to find out which rows that are in the first do not have a match
in the second. If I could create an explicit left join, I could do it by
finding the rows in the second column that are NULL. Here's what I kinda
have, but itjust sits there and never returns anything:

select count(*) from apdf
where apdf.doc_index != ondisk.doc_index;

I tried the NOT IN operator (!!=), but it didn't like that at all. Is !=
a valid operator?

Brett W. McCoy
http://www.lan2wan.com/~bmccoy
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"Biology is the only science in which multiplication means the same
thing as division."