Select all parents or children in same table relation SQL Server

Question

SQL developers, I have a badly planned database as task to learn a lot about SQL Server 2012.

SO, there is the table Elem:

+-----------+----         


        

Answer 1

I've created a function for finding the parents of a specific child where you have to pass the Id of the child.

This will return the list of parents as a comma separated string. Try this if it works for you.

I'm assuming that the parent_key with null value is root.

CREATE FUNCTION checkParent(@childId INT)
RETURNS VARCHAR(MAX)
AS
BEGIN
    DECLARE @parentId VARCHAR(MAX) = NULL
    DECLARE @parentKey INT = null
    SET @parentId = (SELECT parent_key FROM Elem WHERE [KEY] = @childId)

    WHILE(@parentKey IS NOT NULL)
    begin
        SET @parentId = @parentId +  ', ' + (SELECT parent_key FROM Elem WHERE [KEY] = @parentId)
        SET @parentKey = (SELECT parent_key FROM Elem WHERE [KEY] = @parentId)
    END
    RETURN @parentId
END
GO