Oracle中的定义

懒得记了
把参考的链接放这啥时候有缘分啥时候看吧

https://www.icode9.com/content-2-274797.html
https://www.cnblogs.com/klb561/p/11100077.html
http://blog.sina.com.cn/s/blog_8139d7c30100sskd.html
https://www.cnblogs.com/halberd-lee/p/11132535.html
https://www.cnblogs.com/lgx5/p/11290773.html
https://www.bbsmax.com/A/MyJx7E7ezn/
https://blog.csdn.net/wahaa591/article/details/46772769/
https://blog.csdn.net/zhengqiqiqinqin/article/details/9083447
https://www.iteye.com/blog/linglong2110-718129