In one interview, I was asked about the tree structure of the database. At that time, I was a bit blinded. I should have learned the skills myself and found some information on the Internet.
http://blog.csdn.net/tiantiandjava/article/details/45390859
https://my.oschina.net/XYleung/blog/99604
First of all, the database does not have to use a traditional relational database, because the traditional relational library has some functions that are difficult to do or very troublesome to do, especially when the parent-child relationship is particularly complex, at this time we can consider using the NoSql database
http://www.cnblogs.com/huangfox/archive/2012/04/11/2442408.html