为结果集构造行号，实际上是为结果集指定顺序，要排序的话就要知道排序规则：Order By
如：SELECT ROW_NUMBER() OVER(ORDER BY id DESC)AS rownum,
       departname  FROM mdDepartment md
应用：mysql的limit即可使用：
SELECT * FROM
(
SELECT ROW_NUMBER() OVER(ORDER BY id DESC)AS rownum,departname
FROM mdDepartment md
) AS a
WHERE a.rownum>=3 AND a.rownum<=5      --返回3到5行