is a 'pairwise' possible / feasible in SQL?

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Hi, I have a table of the form

aid cid
- ---- -----
1 123
2 456
3 667
3 879
3 123
4 878
4 456
4 123
5 999
5 667
5 879

My goal is to identify for each pair of cid values, the number of
times they have the same aid

Thus for example I would have

pair count
- ---- -----
123 & 456 1
667 & 879 2
...

I currently do this by using a Python script to do a pairwise lookup, as

select count(aid) where cid = 123 and cid = 456;

but I was wondering whether I could construct a single SQL statement
to do this.

Any pointers would be appreciated,

Thanks,
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Rajarshi Guha <rguha(at)indiana(dot)edu>
GPG Fingerprint: D070 5427 CC5B 7938 929C DD13 66A1 922C 51E7 9E84
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All great discoveries are made by mistake.
-- Young

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