Hierarchical queries are statements to retrieve chain of related sets. Its table typically represents the list of linked records like employee-manager sequences. They usually represented in tree structure. SQL standard does not directly support the convenient method for hierarchical data structure since it is born to support the set-based records in table structure.
This problem is also called Transitive closure in the literature http://stencel.mimuw.edu.pl/obd/sbql_rekursja.pdf Object-Oriented Programming with Recursive Queries]. Accordingly several vendors support the extensions. Oracle supports hierarchical queries 1234On Favorites and CONNECT BY. By their SQL extension, the query is much simpler than IBM DB2 Port CONNECT BY to DB2Expanding Recursive Opportunities with SQL UDFs in DB2 v7.2.
Most popular open database, MySQL, however needs an programmatic approaches in two methods:  using additional tables and stored procedures to walk through the graph 567. Thus they use stored procedures 8
For hierarchical trees, various models exist and the comparison reviews also exist: