MySQL - While in SELECT clause - mysql

Question

Is it possible to call a while statement inside a SELECT clause in MySQL ?
Here is a example of what I want to do :
CREATE TABLE `item` (
`id` int,
`parentId` int,
PRIMARY KEY (`id`),
UNIQUE KEY `id` (`id`),
KEY `FK_parentId` (`parentId`),
CONSTRAINT `FK_parentId` FOREIGN KEY (`parentId`) REFERENCES `item` (`id`)
);
I would like to select the root of each item, i.e. the higher ancestor (the item that has no parentId). In my mind, I would do something like this :
select
`id` as 'ID',
while `parentId` is not null do `id` = `parentId` end while as 'Root ID'
from
`item`
Of course this can't work. What is the better way to achieve something like that ?
EDIT
Here a sample data :
id | parentId
1 | NULL
2 | 1
3 | 2
4 | 2
5 | 3
6 | NULL
7 | 6
8 | 7
9 | 7
And expected result :
ID | RootId
1 | NULL
2 | 1
3 | 1
4 | 1
5 | 1
6 | NULL
7 | 6
8 | 6
9 | 6
Thank you.

Answer 1

just use CASE
select
`id` as 'ID',
CASE `parentId` WHEN is not null THEN `parentId` END as 'Root ID'
from
`item`

Answer 2

Here is the procedure:
BEGIN
-- declare variables
DECLARE cursor_ID INT;
DECLARE cursor_PARENTID INT;
DECLARE done BOOLEAN DEFAULT FALSE;
-- declare cursor
DECLARE cursor_item CURSOR FOR SELECT id, parentId FROM item;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
-- create a temporary table
create TEMPORARY table IF NOT EXISTS temp_table as (select id, parentId from item);
truncate table temp_table;
OPEN cursor_item;
item_loop: LOOP
-- fetch row through cursor
FETCH cursor_item INTO cursor_ID, cursor_PARENTID;
IF done THEN
-- end loop if cursor is empty
LEAVE item_loop;
END IF;
-- insert into
insert into temp_table
select MAX(t.id) id, MIN(#pv := t.parentId) parentId
from (select * from item order by id desc) t
join (select #pv := cursor_ID) tmp
where t.id = #pv;
END LOOP;
-- close cursor
CLOSE cursor_item;
-- get the results
SELECT id id, parentid RootId from temp_table order by id ASC;
END
I created a temporary table and kept the results into it while running cursor. I couldn't think of a solution with just one query. I had to go for a cursor.
I took help from the following links:
How to do the Recursive SELECT query in MySQL?
How to create a MySQL hierarchical recursive query