From: "Alfred" <99m@myway.com>
Subject: Tough Problem -- Record Checkouts
Date: 13 Feb 2006 09:33:01 -0800
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To: pgsql-sql@postgresql.org

Imagine a library of books. Each book is a record. The library is the
table. A user may check out a book and the shelf space it once occupied
will store the minute that the user checked the book out. Every 5
minutes, a magical librarian walks through the library and when a book
has been checked out longer than 15 minutes, she has the power to zap
it back out of the user's hands and put it back on the shelf for
someone else. How do you efficiently achieve this in a WHERE clause in
SQL?

For instance, here's a table of several minute columns. CO, in this
case, is the checked out minute. N, in this case, is the current
minute. This translates to, "If the CO = x, and N is within this range,
then clear the CO column."

CO    |     N
------+-----------------------
0     |     15-59
1     |     0, 16-59
2     |     0-1, 17-59
15    |     0-14, 30-59
16    |     0-15, 31-59
30    |     0-29, 45-59
31    |     0-30, 46-59
45    |     0-44
46    |     1-45
59    |     14-58

This becomes some kind of UPDATE statement with a complex WHERE clause.