15일차 # 7-3 ~ 5: Hierarchical Query (계층 질의)

  - a relationship exists between rows in the same table

      => a kind of self-join

  - restriction: SELECT can NOT contain a join or query of join

  - tree walking: null이 나올 때까지

  * Clauses

    - START WITH: starting point of tree walking

                  CAN contain a subquery

                  omitted, all row are considered as root rows

    - CONNECT BY: direction of tree-walking

                  can NOT contain a subquery

    - PRIOR: Parent Row

             can be in connect by and select-list clauses

             can be in either side of condition

  * Hierarchical Query Pseudocolumn

    - LEVEL