Interval type in Oracle

Here are some resources related with Interval:

http://wenku.baidu.com/view/850f72ebe009581b6bd9eb4c.html

http://www.itpub.net/thread-1394807-1-1.html

http://hi.baidu.com/%B3%A4%B1%EC%B5%C4%D6%ED/blog/item/34b911eea29a803dacafd5d1.html

http://wallimn.iteye.com/blog/516843