MySQL: Get Root Node of Parent-Child Structure

Question

I have a table similar to this:

=================
| Id | ParentId |
=================
| 1  | 0        |
-----+-----------
| 2  | 1        |
-----+-----------
| 3  | 0        |
-----+-----------
| 4  | 3        |
-----+-----------
| 5  | 3        |
-----+-----------
| 6  | 0        |
-----+-----------
| 7  | 6        |
-----+-----------
| 8  | 7        |
-----------------

Given an Id, I need to know its root "node" Id. So,

• Given 1, return 1
• Given 2, return 1
• Given 3, return 3
• Given 4, return 3
• Given 5, return 3
• Given 6, return 6
• Given 7, return 6
• Given 8, return 7

There is no limit to the levels of the hierarchy. Is there a SQL that can do what I need?

Answer 1

Actually, you can quite easily do this using a function.

Try running the following .sql script on your favorite empty test database.

--
-- Create the `Nodes` table
--
CREATE TABLE `Nodes` (
     `Id` INT NOT NULL PRIMARY KEY
    ,`ParentId` INT NOT NULL
) ENGINE=InnoDB;

--
-- Put your test data into it.
--
INSERT INTO `Nodes` (`Id`, `ParentId`)
VALUES 
  (1, 0)
, (2, 1)
, (3, 0)
, (4, 3)
, (5, 3)
, (6, 0)
, (7, 6)
, (8, 7);

--
-- Enable use of ;
--
DELIMITER $$

--
-- Create the function
--
CREATE FUNCTION `fnRootNode`
(
    pNodeId INT
)
RETURNS INT
BEGIN
    DECLARE _Id, _ParentId INT;

    SELECT pNodeId INTO _ParentId;

    my_loop: LOOP
        SELECT 
             `Id`
            ,`ParentId`
        INTO 
             _Id
            ,_ParentId
        FROM `Nodes`
        WHERE `Id` = _ParentId;

        IF _ParentId = 0 THEN
            LEAVE my_loop;
        END IF;
    END LOOP my_loop;

    RETURN _Id;
END;
$$

--
-- Re-enable direct querying
--
DELIMITER ;


--
-- Query the table using the function to see data.
--
SELECT 
     fnRootNode(`Nodes`.`Id`) `Root`
    ,`Nodes`.`Id`
    ,`Nodes`.`ParentId`
FROM `Nodes`
ORDER BY 
    fnRootNode(`Nodes`.`Id`) ASC
;

-- EOF

Output will be:

Root Id   ParentId
==== ==== ========
1    1    0
1    2    1
3    3    0
3    4    3
3    5    3
6    6    0
6    7    6
6    8    7