mysql - 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 (id, parent_id, name) VALUES
(1, 0, 'pizza'),        --node 1
(2, 0, 'burger'),       --node 2
(3, 0, 'coffee'),       --node 3
(4, 1, 'piperoni'),     --node 1.1
(5, 1, 'cheese'),       --node 1.2
(6, 1, 'vegetariana'),  --node 1.3
(7, 5, 'extra cheese'); --node 1.2.1

To sort it hierarchically by id or name:
'pizza' //node 1
'piperoni' //node 1.1
'cheese' //node 1.2
'extra cheese' //node 1.2.1
'vegetariana' //node 1.3
'burger' //node 2
'coffee' //node 3

EDIT: the number at the end of the name is to visualize the strucutre better, it is not for sorting.

EDIT 2: as mentioned several times ... the number at the end of the name "cheese 1.2" was only for visualization purpose, NOT for sorting. I moved them as comments, too many people got confused, sorry.

Answer 1

By adding a path column and a trigger, this can be done fairly easily.

First add a varchar column that will contain the path from root to the node:

ALTER TABLE category ADD path VARCHAR(50) NULL;

Then add a trigger that calculates the path on insert:

(simply concats the new id with path of the parent)

CREATE TRIGGER set_path BEFORE INSERT ON category
  FOR EACH ROW SET NEW.path = 
  CONCAT(IFNULL((select path from category where id = NEW.parent_id), '0'), '.', New.id);

Then simply select order by path:

SELECT name, path FROM category ORDER BY path;

Result:

pizza         0.1
piperoni      0.1.4
cheese        0.1.5
extra cheese  0.1.5.7
vegetariana   0.1.6
burger        0.2
coffee        0.3

See fiddle.

This way maintenance cost is also minimal. The path field is hidden when inserting and is calculated via trigger. Removing a node has no overhead, since all the children of the node are also removed. The only problem is when updating the parent_id of a node; Well, don't do that! :)