exists 使用

1、exists 使用

https://www.jianshu.com/p/cfdb70368589 
https://www.cnblogs.com/mytechblog/articles/2105785.html 
https://www.cnblogs.com/netserver/archive/2008/12/25/1362615.html 
https://blog.csdn.net/qq_33556442/article/details/89174484?depth_1-utm_source=distribute.pc_relevant.none-task&utm_source=distribute.pc_relevant.none-task
https://blog.csdn.net/kevinzhaoyp/article/details/50948653      
https://blog.csdn.net/a1275302036/article/details/78885032