Question

Imagine a table like below:

Table : Person

Fields: ID, FullName, ParentID

Where ParentID references an ID in Person table again.

Question:
For the example given above, how can I query in TSQL to find the toppest Parent of the Person with ID = 7 ?

Answer 1

You would use common table expressions and do a recursive query until you find the topmost parent like so:

DECLARE @id INT
SET @id = 5 

CREATE TABLE #tmp (id INT , ParentId INT)
INSERT INTO #tmp VALUES(1,0)
INSERT INTO #tmp VALUES(2,1)
INSERT INTO #tmp VALUES(3,2);
INSERT INTO #tmp VALUES(4, 3);
INSERT INTO #tmp VALUES(5,4);

    WITH parent AS
    (
        SELECT id, parentId  from #tmp WHERE id = @id
        UNION ALL 
        SELECT t.id, t.parentId FROM parent
        INNER JOIN #tmp t ON t.id =  parent.parentid
    )

    SELECT TOP 1 id FROM  parent
    order by id asc

DROP TABLE #tmp