Sunday, September 12 2010
PostgreSQL, YeSQL

Window Functions example remix

The drawback of hosting a static only website is, obviously, the lack of comments. What happens actually, though, is that I receive very few comments by direct mail. As I don't get another spam source to cleanup, I'm left unconvinced that's such a drawback. I still miss the low probability of seeing blog readers exchange directly, but I think a mailing list would be my answer, here...

Anyway, David Fetter took the time to send me a comment by mail with a cleaned up rewrite of the previous entry SQL, here's it for your pleasure!

        o, w,
        CASE WHEN
            LAG(w) OVER(w) IS DISTINCT FROM w AND
            ROW_NUMBER() OVER (w) > 1 /* Eliminate first change */
        THEN 1
        END AS change
    FROM (
            (1, 5),
            (2, 10),
            (3, 7),
            (4, 7),
            (5, 7)
    ) AS data(o, w)
    WINDOW w AS (ORDER BY o) /* Factor out WINDOW */
SELECT SUM(change) FROM t;

As you can see David chose to filter the first change in the subquery rather than hacking it away with a simple -1 at the outer level. I'm still wondering which way is cleaner (that depends on how you look at the problem), but I think I know which one is simpler! Thanks David for this blog entry!