How to do MySQL Looped Join which tests if results are complete?

Question

Situation:
I have a mysql table of directories. Each directory has a parent directory (stored as parentID), up to the point where the root directory has a parentID of

Answer 1

Well, It could be you did not find a good web example, because you did use the wrong search-term. The problem described fits perfectly into oracles CONNECT BY PRIOR statement and by googling for mysql-equivalents to this statement you find http://explainextended.com/2009/03/17/hierarchical-queries-in-mysql/ quite fast.

Because its not that easy (and I dont have a mysql-db to rape here) to write this stuff, just have a glance at the good examples given (you can even do that without deployed functions via http://explainextended.com/2009/07/20/hierarchical-data-in-mysql-parents-and-children-in-one-query/.

If you still dont get the hang of those, I might be able to help at home.

Answer 2

Alright, found the time to actually deploy a simple database with a similar structure as described.

The table is the following:

CREATE TABLE `t_hierarchy` (
    `rowID` INT(11) NULL DEFAULT NULL,
    `name` VARCHAR(50) NULL DEFAULT NULL COLLATE 'latin1_general_ci',
    `parentID` INT(11) NULL DEFAULT NULL
);

I basicly inserted the exact same stuff as you have given above but used NULL values instead of 0 for root/no parent

What I've done is the quite cryptic example from http://explainextended.com/2009/07/20/hierarchical-data-in-mysql-parents-and-children-in-one-query/ . and just corrected the column names to fit mine.

Since this only generates you a recursive hierarchy, I just added a stupid join into the example ( ad.rowID = qi.id ):

 SELECT  qi.id, qi.parent, ad.rowId, ad.name, level
FROM    (
        SELECT  @r AS id,
                (
                SELECT  @r := parentID
                FROM    t_hierarchy
                WHERE   rowID = id
                ) AS parent,
                @l := @l + 1 AS level
        FROM    (
                SELECT  @r := 5, -- change this 5 to the directory ID you want to resolve
                        @l := 0,
                        @cl := 0
                ) vars,
                t_hierarchy h
        WHERE   @r <> 0
        ORDER BY
                level DESC
        ) qi, t_hierarchy ad
        WHERE ad.rowID = qi.id

And this generates the follwoing (desired) output:

id parent rowId name level

1 NULL 1 Dir1 3

3 1 3 Subdir1 2

5 3 5 Subdir3 1

Level is a helper column which tells you how "deep" it had to resolve to reach this. All you will have to do is change the "5" next to @r := to the directory ID from where you wanna iterate down.

If you want to switch the direction (from up to down) simply sort by level column ([...] WHERE ad.rowID = qi.id ORDER BY level ASC )

Hope this helps you out.

Edit: qi.id and ad.rowID are duplicates, just remove one of them ;-)... damn I hate that hierarchy stuff