谁帮我解释一下这段查询语句啊. 越详细越好

[zhulinjia]

select bm_URL
	    from bookmark
	    where username in 
	   	 (select distinct(b2.username)
                  from bookmark b1, bookmark b2
		  where b1.username='$valid_user'
                  and b1.username != b2.username
                  and b1.bm_URL = b2.bm_URL)
	    and bm_URL not in  (select bm_URL
			      from bookmark
			      where username='$valid_user')
            group by bm_url
            having count(bm_url)>$popularity";

[飘零一叶]

这个该如何解释....

[叶子]

distinct --过滤掉重复的

$valid_user,$popularity --是两个变量

group by bm_url --按照bm_url进行分组

[dawugui]

........