hierarquia mysql auto-junção, recuperar todas as subcategorias

Esquema:

create table category
(   category_id int not null auto_increment primary key,
    category_name varchar(40) not null,
    parent_id int null,  -- index on this column not a shabby idea
    unique key (category_name)
);

insert category(category_name,parent_id) values ('car',null),('food',null); -- 1,2
insert category(category_name,parent_id) values ('ford',1),('chevy',1),('fruit',2); -- 3,4,5
insert category(category_name,parent_id) values ('economy',3),('escort',6),('exhaust',7); -- 6,7,8
insert category(category_name,parent_id) values ('chassis',7),('loud',8),('banana',5); -- 9,10,11
-- ok granted I could have explicity inserted category_id to make it more obvious

Criar procedimento armazenado:

-- drop procedure showHierarchyBelow;
delimiter $$
create procedure showHierarchyBelow
(
catname varchar(40)
)
BEGIN
    -- deleteMe parameter means i am anywhere in hierarchy of role
    -- and i want me and all my offspring deleted (no orphaning of children or theirs)
    declare bDoneYet boolean default false;
    declare working_on int;
    declare theCount int;
    declare findFirst int;

    select ifnull(category_id,0) into findFirst from category where category_name=catname;

    CREATE TABLE xx_RecursishHelper_xx
    (   -- it's recurshish, not recursive
        category_id int not null,
        processed int not null
    );
    if isnull(findFirst) then
        set findFirst=0;
    end if;
    insert into xx_RecursishHelper_xx (category_id,processed) select findFirst,0;
    if (findFirst=0) then
        set bDoneYet=true;
    else
        set bDoneYet=false;
    end if;

    while (!bDoneYet) do
        -- I am not proud of this next line, but oh well
        select count(*) into theCount from xx_RecursishHelper_xx where processed=0;

        if (theCount=0) then 
            -- found em all
            set bDoneYet=true;
        else
            -- one not processed yet, insert its children for processing
            SELECT category_id INTO working_on FROM xx_RecursishHelper_xx where processed=0 limit 1;
            insert into xx_RecursishHelper_xx (category_id,processed)
            select category_id,0 from category
            where parent_id=working_on;

            -- mark the one we "processed for children" as processed
            update xx_RecursishHelper_xx set processed=1 where category_id=working_on;
        end if;
    end while;

    delete from xx_RecursishHelper_xx where category_id=findFirst;

    select x.category_id,c.category_name
    from xx_RecursishHelper_xx x
    join category c
    on c.category_id=x.category_id;

    drop table xx_RecursishHelper_xx;
END
$$

Testar proc armazenado:

call showHierarchyBelow('food');
+-------------+---------------+
| category_id | category_name |
+-------------+---------------+
|           5 | fruit         |
|          11 | banana        |
+-------------+---------------+

call showHierarchyBelow('car');
+-------------+---------------+
| category_id | category_name |
+-------------+---------------+
|           3 | ford          |
|           4 | chevy         |
|           6 | economy       |
|           7 | escort        |
|           8 | exhaust       |
|           9 | chassis       |
|          10 | loud          |
+-------------+---------------+

call showHierarchyBelow('ford');
+-------------+---------------+
| category_id | category_name |
+-------------+---------------+
|           6 | economy       |
|           7 | escort        |
|           8 | exhaust       |
|           9 | chassis       |
|          10 | loud          |
+-------------+---------------+

call showHierarchyBelow('xxx');
-- no rows

Observe