Re: Self Join Help

Presto ->
select a.topicname as parent, b.topicname as child from topics as a left
join topics as b on a.id = b.pid where a.pid = 0;
 parent |   child
--------+------------
 Foo    |
 Apache | PHP
 XHTML  |
 News   | World News
 News   | Tech News
(5 rows)

Thanks for pointing me in the right direction....

Gerard Samuel wrote:

> Thanks for you help thus far.
> The final goal would be to achieve results like ->
>   parent   |   child
> ------------+------------
> Foo        |
> Apache     | PHP
> XHTML      |
> News       | Tech News
> News       | World News
>
> Im playing with the SQL to see if its possible, but any insight would
> be appreciated.
> Thanks again.
>
> apz wrote:
>
>> this is second time I post to a forum, and second time I correct
>> myself... ugh, I should delay posting by 15 minutes, or stop
>> re-reading my emails after posting... ;D
>>
>> anyways:
>>
>> apz wrote:
>>
>>> test=# select a.topicname as parent, b.topicname as child from
>>> topics as a left join topics as b on a.id = b.pid;
>>>
>>>  parent |   child
>>> --------+------------
>>>  XHTML  | null
>>>  Foo    | null
>>>  Apache | PHP
>>>  News   | Tech News
>>>  News   | World News
>>
>>
>>
>> this actually should return:
>>
>>
>> test=# select a.topicname as parent, b.topicname as child from topics as
>> a left join topics as b on a.id = b.pid;
>>
>>  parent     | child
>> ------------+------------
>>  XHTML      | null
>>  Foo        | null
>>  Apache     | PHP
>>  News       | Tech News
>>  News       | World News
>>  PHP        | null
>>  Tech News  | null
>>  World News | null
>>
>>
>> the querry with left join should return child=null if a node is a leaf.
>> so XHTML and PHP return child as null because neither have any nodes
>> underneath.
>> this also should mean that topicname should not allow null values,
>> not to confuse ourselves further on.
>>
>> so two ways are:
>>  - add one node which is always root, use your querry
>>  - use left join, when no child then child returns as null
>>
>>
>>
>> /apz,  You can always tell luck from ability by its duration.
>>
>>
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