mysql常见用法

https://blog.csdn.net/weixin_38820375/article/details/87864624     多个字段分组解释

https://www.jb51.net/article/124512.htm

 

 

https://www.cnblogs.com/xiaozizi/p/5816802.html    mysql中的！=null与is not null区别

 

https://www.cnblogs.com/areyouready/p/7588258.html   mysql中的case语句

 

https://blog.csdn.net/k8080880/article/details/53411782/ mysql中的if语句