SQL以"循环"顺序返回行

Question

我有一堆存储在表中的URL,等待脚本删除.但是,其中许多网址来自同一网站.我想以"网站友好"的顺序返回这些网址(也就是说,尝试避免连续存在来自同一网站的两个网址),这样我就不会在短时间内通过制作过多的http请求而意外阻止.

数据库布局是这样的:

create table urls (
    site varchar,       -- holds e.g. www.example.com or stockoverflow.com
    url varchar unique
);

示例结果:

SELECT url FROM urls ORDER BY mysterious_round_robin_function(site);

http://www.example.com/some/file
http://stackoverflow.com/questions/ask
http://use.perl.org/
http://www.example.com/some/other/file
http://stackoverflow.com/tags

我想到了像" ORDER BY site <> @last_site DESC" 这样的东西,但我不知道如何写这样的东西.

Answer 1

有关其工作原理的更详细说明，请参阅我博客中的这篇文章：

• PostgreSQL：循环顺序

新的PostgreSQL 8.4：

SELECT  *
FROM    (
        SELECT  site, url, ROW_NUMBER() OVER (PARTITION BY site ORDER BY url) AS rn
        FROM    urls
        )
ORDER BY
        rn, site

对于旧版本：

SELECT  site,
        (
        SELECT  url
        FROM    urls ui
        WHERE   ui.site = sites.site
        ORDER BY
                url
        OFFSET  total
        LIMIT   1
        ) AS url
FROM    ( 
        SELECT  site, generate_series(0, cnt - 1) AS total
        FROM    (
                SELECT  site, COUNT(*) AS cnt
                FROM    urls
                GROUP BY
                        site
                ) s
        ) sites
ORDER BY
        total, site

，尽管它可能效率较低。