select datediff(ward_hd,allen_hd)from (select hiredate as ward_hdfrom empwhere ename=‘WARD‘) x,(select hiredate as allen_hdfrom empwhere ename=‘ALLEN‘)y;
8.02 计算两个日期之间的天数
原文：http://www.cnblogs.com/liang545621/p/7523295.html
