- https://sqlzoo.net/wiki/SELECT_basics/zh
- SQL statements online practice http://sample.jimstone.com.cn/xsql/
- SQL statement to test http://www.w3school.com.cn/quiz/quiz.asp?quiz=sql
- sql statement Exercise 50 questions (Mysql version) https://blog.csdn.net/fashion2014/article/details/78826299/
- http://sqlfiddle.com/