sql语句

https://www.runoob.com/sql/sql-join-left.html

https://www.cnblogs.com/bad-robot/p/9788959.html

https://www.jianshu.com/p/11a048d5ea4f

https://www.w3school.com.cn/sql/sql_join_inner.asp

https://www.cnblogs.com/sugartang/p/10967435.html

菜鸟教程Mysql

https://www.cnblogs.com/bluedy1229/p/8992965.html

https://baijiahao.baidu.com/s?id=1589166609744115558&wfr=spider&for=pc

https://www.cnblogs.com/rgever/p/9335075.html

https://www.cnblogs.com/sueris/p/6650301.html

https://zhidao.baidu.com/question/294123165.html

https://www.cnblogs.com/cangqinglang/p/10081563.html

https://www.cnblogs.com/liuri/p/9379468.html