问题
I am facing an issue while using the CONNECT BY
clause in Oracle for finding hierarchical data. Let me give an example: A is my parent part which has child part B and B also has a child part C. When I am using the CONNECT BY
clause I am able to get all the three levels but I only want the top most level, i.e. A.
回答1:
Oracle has a LEVEL pseudocolumn that you can use:
SELECT
myTable.ID,
myTable.ParentID
FROM myTable
WHERE LEVEL = 1
CONNECT BY PRIOR myTable.ID = myTable.ParentID
To find a top-level (root) value from any level, precede the column name with the CONNECT_BY_ROOT
operator:
SELECT
myTable.ID,
myTable.ParentID,
CONNECT_BY_ROOT myTable.ID AS "Top Level ID"
FROM myTable
CONNECT BY PRIOR myTable.ID = myTable.ParentID
回答2:
I am adding this solution for tables with one or multiple trees (hierarchical data).
Starting with one node (row) somewhere in the tree (hierarchical data), wanting to find the top node (root).
The query is taking advantage of the fact that the ONLY the top node (root) of a tree don't have a parent, which is a very common attribute of the top node (root) in any tree structure.
SELECT
c.id
FROM
node c
WHERE
c.parent_id is null
CONNECT BY PRIOR
c.parent_id = c.id
START WITH
c.id = 1059002615
回答3:
SELECT * FROM (
SELECT CONNECT_BY_ROOT myTable.ID AS "Top Level ID"
FROM myTable
CONNECT BY PRIOR myTable.ID = myTable.ParentID
)
WHERE myTable.ParentID IS NULL
来源:https://stackoverflow.com/questions/18110471/connect-by-clause-to-get-the-top-of-hierarchy