Fun with SQL

Folks,

I just made some fun and interesting use of 'tricky" SQL I thought I
would share, as it gives an example of both clever SQL and function
usage.

THE PROBLEM: One of my clients is reporting skipped invoice numbers in
their database. At a glance, I can't find any. While the client uses
numerical invoice numbers, invoice_no is a VARCHAR field to accomodate
other schemes. How can I check for ALL skipped invoice numbers in the
database? Searching for data that is *not* there, without a reference
list to compare, is a challenge.

THE ANSWER: Look for any invoice number that is not succeeded by the
invoice number which is its immediate numerical successor. I will
also have to do some type conversions to do math on a VARCHAR field.

THE QUERY:
SELECT (to_number(invoice_no, '99999')::INT4 + 1::INT4) as skipped
FROM invoices
WHERE ltrim(to_char((to_number(invoice_no, '99999')::INT4 + 1::INT4),
'99999'))
NOT IN (SELECT invoice_no FROM invoices);

WHAT I GOT:
skipped
-------
10519
10839

Note that 10839 is the *next* invoice number to be generated, and as
such, not skipped.

WHAT I TOLD THE CLIENT: "Hmmm, I can only see one skipped invoice in
the last 350 you've run. This does not seem to be a chronic problem."

-Josh Berkus

______AGLIO DATABASE SOLUTIONS___________________________
Josh Berkus