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Hi,

I'm playing around with putting a hierarchy of items into the database. But for some reason I'm having a mental block understanding the following:

I have a table category with id and parent_id implementing the typical adjacency model.

To get the first two levels of the hierarchy I use:

SELECT t1.name as lev1, t2.name as lev2
FROM category t1
LEFT JOIN category t2 ON t2.parent = t1.id
WHERE t1.name = 'ROOT'
;

Now what I don't understand is that the root node (which has a NULL parent_id) is not selected.

My understanding from the outer join is that it would return all items from the "left" tables regardless whether they have a corresponding row in the "right" table.

So given the data

name, id, parent_id
ROOT, 1, NULL
CHILD1, 2, 1
CHILD2, 3, 1

I would have expected the following result:

ROOT, NULL
ROOT, CHILD1
ROOT, CHILD2

but the row with (ROOT,NULL) is not returned.

I'm sure I'm missing something very obvious, but what?

Thanks in advance
Thomas


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On Mon, Apr 21, 2008 at 03:48:23PM +0200, Thomas Kellerer wrote:
> name, id, parent_id
> ROOT, 1, NULL
> CHILD1, 2, 1
> CHILD2, 3, 1
>
> I would have expected the following result:
>
> ROOT, NULL
> ROOT, CHILD1
> ROOT, CHILD2
>
> but the row with (ROOT,NULL) is not returned.

why would you expect it?
the columns are: parent and child (on your output).
you dont have any row that has *parent_id = 1* and id = NULL.

you can get this output though:

NULL, ROOT
ROOT, CHILD1
ROOT, CHILD2

with this query:

select p.name as parent, c.name as child from category c left outer join category p on c.parent_id = p.id

depesz

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hubert depesz lubaczewski, 21.04.2008 16:05:
>> ROOT, 1, NULL
>> CHILD1, 2, 1
>> CHILD2, 3, 1
>>
>> I would have expected the following result:
>>
>> ROOT, NULL
>> ROOT, CHILD1
>> ROOT, CHILD2
>>
>> but the row with (ROOT,NULL) is not returned.
>
> why would you expect it?
> the columns are: parent and child (on your output).
> you dont have any row that has *parent_id = 1* and id = NULL.

Ah, of course that's where my mental block was
Thanks for the quick response

> you can get this output though:
>
> NULL, ROOT
> ROOT, CHILD1
> ROOT, CHILD2
>
> with this query:
>
> select p.name as parent, c.name as child from category c left outer join category p on c.parent_id = p.id

If the table contains more levels (i.e. child1 being the parent of another item) this bring others back as well. And I was trying to retrieve the full path for each item (I do know the max. number of levels)

Cheers, and thanks a lot for the quick response
Thomas


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> If the table contains more levels (i.e. child1 being the parent of
> another item) this bring others back as well. And I was trying to
> retrieve the full path for each item (I do know the max. number of levels)
>

There is no max number of levels. I don't try to pick all the levels
out in one query though. In my case, since I don't know the number of
levels, I do the query recursively. This works just fine to build a
complex tree but you might consider caching the results.

-Dennis