order sql tree hierarchy

Question

What is the best way to sort a table like this:

CREATE TABLE category(
    id INT(10),
    parent_id INT(10),
    name VARCHAR(50)
);

INSERT INTO category (         


        

Answer 1

I think everyone is over-architect-ing the solution. If your goal is really represented by your example, as in 3-levels with the virtual top level of 0 id, this should suffice.

SELECT *
     , id AS SORT_KEY
  FROM category a
 WHERE parent_id = 0
UNION ALL
SELECT a.*
     , CONCAT(b.id, '.', a.id) AS SORT_KEY
  FROM category a
     , category b
 WHERE b.parent_id = 0
   and b.id = a.parent_id
UNION ALL
SELECT a.*
     , CONCAT(c.id,'.', b.id,'.', a.id) AS SORT_KEY
  FROM category a
     , category b
     , category c
 WHERE c.parent_id = 0
   and b.id = a.parent_id
   AND c.id = b.parent_id
ORDER BY sort_key