SQL知识总结

case when
http://blog.sina.com.cn/s/blog_4c538f6c01012mzt.html
NVL NVL2 NULLIF Coalesce
https://www.cnblogs.com/zrui-xyu/p/4819712.html
join union
https://www.cnblogs.com/logon/p/3748020.html