Re: LinkedList

Ray,

There's a good introductory article on Sitepoint for doing this kind of thing:

http://www.sitepoint.com/article/hierarchical-data-database

The "Modified Preorder Tree Traversal" is quite a nice method that I
would expect to be a magnitude faster than recursive joins, although
does have some drawbacks.

Kind Regards,

Neil.

On 4/27/06, Jim C. Nasby <jnasby(at)pervasive(dot)com> wrote:
> decibel=# select * from t;
> a | b
> ---+---
> 1 | 0
> 3 | 1
> 5 | 3
> 7 | 5
> 2 | 0
> 4 | 2
> 6 | 4
> 8 | 6
> (8 rows)
>
> decibel=# select * from t x join t y on(x.a=y.b) where y.a=7;
> a | b | a | b
> ---+---+---+---
> 5 | 3 | 7 | 5
> (1 row)
>
> decibel=# select * from t x join t y on(x.a=y.b) where y.a=8;
> a | b | a | b
> ---+---+---+---
> 6 | 4 | 8 | 6
> (1 row)
>
> decibel=#
>
> As you can see, it selects the right data, but you'll need to step
> through it somehow. You might be able to do it with a generate_series(),
> or you can use a function. If we get WITH support/recursion in 8.2 you'd
> use that.
>
> I think that "SQL For Smarties" by Joe Celko might have an example of
> how to do this without using a function. Even if it doesn't it's a book
> any serious database developer should own.
>
> On Wed, Apr 26, 2006 at 10:35:15AM -0700, Ray Madigan wrote:
> > Scott,
> >
> > Thanks for your reply, I tried what you said, worked around a few things
> > but I am still stuck. The main reason is I didn't do an adequate job of
> > explaining the situation. The table implements many linked lists and I want
> > to traverse one of them given the end of the list.
> >
> > Say the table contains
> >
> > h | v | j
> > 1 0 100
> > 3 1 300
> > 5 3 500
> > 7 5 700
> >
> > 2 0 200
> > 4 2 400
> > 6 4 600
> > 8 6 800
> >
> > If I specify t.h = 8 I want to traverse the even part of the table
> > If I specify t.h = 7 I want to traverse the odd part of the table
> >
> > If you can send me to a book to read I am willing
> >
> > Thanks
> >
> > -----Original Message-----
> > From: pgsql-sql-owner(at)postgresql(dot)org
> > [mailto:pgsql-sql-owner(at)postgresql(dot)org]On Behalf Of Scott Marlowe
> > Sent: Wednesday, April 26, 2006 8:59 AM
> > To: Ray Madigan
> > Cc: pgsql-sql(at)postgresql(dot)org
> > Subject: Re: [SQL] LinkedList
> >
> >
> > On Wed, 2006-04-26 at 11:09, Ray Madigan wrote:
> > > I have a table that I created that implements a linked list. I am not an
> > > expert SQL developer and was wondering if there are known ways to traverse
> > > the linked lists. Any information that can point me in the direction to
> > > figure this out would be appreciated. The table contains many linked
> > lists
> > > based upon the head of the list and I need to extract all of the nodes
> > that
> > > make up a list. The lists are simple with a item and a link to the
> > history
> > > item so it goes kind of like:
> > >
> > > 1, 0
> > > 3, 1
> > > 7, 3
> > > 9, 7
> > > ...
> > >
> > > Any suggestions would be helpful, or I will have to implement the table
> > > differently.
> >
> > You should be able to do this with a fairly simple self-join...
> >
> > select a.id, b.aid, a.field1, b.field1
> > from mytable a
> > join mytable b
> > on (a.id=b.aid)
> >
> > Or something like that.
> >
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>
> --
> Jim C. Nasby, Sr. Engineering Consultant jnasby(at)pervasive(dot)com
> Pervasive Software http://pervasive.com work: 512-231-6117
> vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461
>
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