参考资料：
https://www.cnblogs.com/kubixuesheng/p/4404004.html https://www.cnblogs.com/Evsward/p/dag.html https://blog.csdn.net/qq_34037046/article/details/102710052 https://blog.csdn.net/oDaiLiDong/article/details/85564366
Resources:
[DAGSQL1] http://dgoyani.blogspot.com/2005/09/effectivewaytoexpanddatatreein.html
[DAGSQL2] http://www.dbazine.com/oracle/orarticles/tropashko4
[DAGSQL3] http://dev.mysql.com/techresources/articles/hierarchicaldata.html
[DAGSQL4] http://www.codeproject.com/KB/database/persisting_trees.aspx
[DAGSQL5] http://www.yafla.com/papers/sqlhierarchies/sqlhierarchies2.htm
[DAGSQL6] http://www.artfulsoftware.com/mysqlbook/sampler/mysqled1ch20.html
[DAGSQL7] http://stackoverflow.com/questions/317322/optimizedsqlfortreestructures
[DAGSQL8] http://www.codeproject.com/KB/database/Modeling_DAGs_on_SQL_DBs.aspx
[DAGSQL9] http://www.adpgmbh.ch/ora/sql/connect_by.html
[DAGSQL10] http://www.postgresql.org/docs/current/static/ltree.html
