https://blog.csdn.net/weibo1230123/article/details/90344535
1. http://sample.jimstone.com.cn/xsql/
2. http://sqlfiddle.com/
3. https://sqlbolt.com/lesson/select_queries_introduction
4. https://www.w3resource.com/sql-exercises/sql-retrieve-from-table.php
在线的mysql练习 - 在线SQL 廖雪峰的
https://www.liaoxuefeng.com/wiki/1177760294764384/1217864791925600
sql语句练习50题(Mysql版) https://blog.csdn.net/fashion2014/article/details/78826299/