Making a recursive CTE like query, but for many records

Question

I have a table with a hierarchy ( with a recursive parentID column), for one record, I know how to get the last parent record, with the following code :

declare @Id integer

;WITH CTE AS
(
    SELECT  a.[Id],
            a.[ParentId]
    FROM    [Area] a WITH (NOLOCK)
    where a.[Id] = @Id 

    UNION ALL

    SELECT  a.[Id],
            a.[ParentId]
    FROM [Area] a WITH (NOLOCK)
    INNER JOIN CTE cte ON cte.[ParentId] = a.[Id]
)

SELECT top 1 a.[Id]
FROM CTE a
WHERE a.ParentId is null

It works as long as I'm working with one id, here @Id.

But I can't find a way to do the same but for a list of Ids, without using cursors, as CTE seems to be OK only when y'oure working on one record.

Could someone point me at the right direction ?

Many thanks

Answer 1

Assuming the original table design is somewhat like this:

CREATE TABLE dbo.Area
(
    RowID integer PRIMARY KEY,
    GroupID  integer NOT NULL,
    ParentID integer NULL,
);

Sample data:

INSERT dbo.Area
    (RowID, GroupID, ParentID)
VALUES 
    (1, 1, NULL), -- Root
    (2, 1, 1),
    (3, 1, 2),
    (4, 2, NULL), -- Root
    (5, 2, 4),
    (6, 2, 5),
    (7, 2, 6);

The following recursive CTE query finds roots for the RowID list given in the anchor:

WITH R AS
(
    SELECT
        A.RowID,
        A.RowID AS OriginalRowID,
        A.ParentID
    FROM dbo.Area AS A
    WHERE
        A.RowID IN (3, 7)

    UNION ALL

    SELECT
        A.RowID,
        R.OriginalRowID,
        A.ParentID
    FROM dbo.Area AS A
    JOIN R
        ON R.ParentID = A.RowID
)
SELECT
    R.OriginalRowID,
    R.RowID
FROM R
WHERE
    R.ParentID IS NULL
ORDER BY
    R.OriginalRowID
OPTION (MAXRECURSION 0);

The output shows each supplied RowID and the root for that group: