Recursive query tree structure __oracle in Oracle tables

If you store a tree structure in a table, you need to deposit two field IDs and ParentID in the tables, indicating who the parent of each record is.

Table:treetable

Id	ParentID	Field 3	......
1	Null	A	......
2	1	B	......
3	2	D	......

Select ... can be used in Oracle. From ... Start with (starting node) ... CONNECT by ... PRIOR (indicates previous record) ... (Order siblings by levels) clause implements a recursive query tree structure.

1. Traverse the child node from the root node.: (A whole tree)

SELECT * from treetable start and id=1 connect by prior Id=parentid (prior represents previous record)

2. Traced from one leaf to the root node: (one branch of the tree)

SELECT * from treetable start and id=3 connect by prior Parentid=id (prior represents previous record)

3. The level keyword can be queried.

Select *,level from treetable start and id=1 connect by prior Id=parentid (prior represents the previous record)