I have a table called 'tasks' with a structure like this:

Taskid ParentId
----------------------------
1 null
2 1
3 1
4 Null
5 4
6 4
7 Null
8 Null
9 15
10 18


The taskid column is the primary key and the parentid column references
values in the taskid column. However, it is possible for the parentid to be
a value that is not found in the taskid column (hence my question.). I need
to write a query that will retrieve all rows where the parentid is either
'null' or does not exist in the taskid column.

Any help would really be appreciated.

Thanks!

Kevin

On 2/22/06 9:43 AM, "Kevin Crenshaw" <kcrenshaw@viscient.com> wrote:

> I have a table called 'tasks' with a structure like this:
>
> Taskid ParentId
> ----------------------------
> 1 null
> 2 1
> 3 1
> 4 Null
> 5 4
> 6 4
> 7 Null
> 8 Null
> 9 15
> 10 18
>
>
> The taskid column is the primary key and the parentid column references
> values in the taskid column. However, it is possible for the parentid to be
> a value that is not found in the taskid column (hence my question.). I need
> to write a query that will retrieve all rows where the parentid is either
> 'null' or does not exist in the taskid column.

How about:

Select * from tasks where parentid is null or parentid not in (select
distinct(taskid) from tasks);




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That did the trick. Thanks for your help!



-----Original Message-----
From: pgsql-novice-owner@postgresql.org
[mailtogsql-novice-owner@postgresql.org] On Behalf Of Sean Davis
Sent: Wednesday, February 22, 2006 10:03 AM
To: Kevin Crenshaw; pgsql-novice@postgresql.org
Subject: Re: [NOVICE] SQL Question




On 2/22/06 9:43 AM, "Kevin Crenshaw" <kcrenshaw@viscient.com> wrote:

> I have a table called 'tasks' with a structure like this:
>
> Taskid ParentId
> ----------------------------
> 1 null
> 2 1
> 3 1
> 4 Null
> 5 4
> 6 4
> 7 Null
> 8 Null
> 9 15
> 10 18
>
>
> The taskid column is the primary key and the parentid column references
> values in the taskid column. However, it is possible for the parentid to
be
> a value that is not found in the taskid column (hence my question.). I
need
> to write a query that will retrieve all rows where the parentid is either
> 'null' or does not exist in the taskid column.

How about:

Select * from tasks where parentid is null or parentid not in (select
distinct(taskid) from tasks);




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