Re: How to 'normalise' this scenario

From: Fred. <ghrno-google_at_yahoo.com>
Date: Wed, 18 May 2011 06:27:23 -0700 (PDT)
Message-ID: <2b14fcff-c657-446b-ae57-1cc7609a685a_at_p23g2000vbl.googlegroups.com>



On May 18, 7:14 am, Erwin <e.sm..._at_myonline.be> wrote:
> On 17 mei, 21:50, "Fred." <ghrno-goo..._at_yahoo.com> wrote:
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> > On May 17, 11:16 am, Erwin <e.sm..._at_myonline.be> wrote:
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> > > On 17 mei, 13:16, Frank Millman <fr..._at_chagford.com> wrote:
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> > > > On May 17, 12:08 pm, Erwin <e.sm..._at_myonline.be> wrote:
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> > > > > On 16 mei, 08:50, Frank Millman <fr..._at_chagford.com> wrote:
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> > > > Hi Erwin
>
> > > > Thanks for your comments - you have got me thinking.
>
> > > > I have stripped most of the comments, because what I have left is the
> > > > crux of the matter.
>
> > > > > You have not stated what "the requirement" is.
>
> > > > Let's take an example we should all be familiar with - a file system.
>
> > > > Assume that we have a lot of files, and we have a table where each row
> > > > represents one file, with columns such as file name, disk address,
> > > > date created, date last modified, etc.
>
> > > > You can sort the table by name, creation date, etc. However, it is
> > > > getting difficult to manage, so a bright spark comes up with the idea
> > > > of directories/folders.
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> > > > The user can create directories on demand, place directories within
> > > > directories, assign files to directories, move files between
> > > > directories, etc.
>
> > > > The requirement is to create a database structure to represent the
> > > > directories.
>
> > > > My guess is that you could use an adjacency list or a nested set for
> > > > this purpose. However, where I am getting stuck is, how does one link
> > > > an entry in the 'files' table to an entry in the 'directories' table,
> > > > to indicate that a particular file resides in a particular directory.
>
> > > > I have been trying to create an entry in the 'directories' table that
> > > > represents, or points to, a 'file'. From your comments, it would
> > > > appear that this is the wrong approach.
>
> > > > I could carry on and speculate further, but I think this is a good
> > > > time to pause and ask if this is a good example, and ask if there is a
> > > > preferred solution.
>
> > > > TIA
>
> > > > Frank
>
> > > That you're now thinking is a good thing :-)
>
> > > The contents of file systems are not the best example for discussing
> > > management of graph data, because the "identity" (the means for
> > > identification) of a file typically is a combination (concatenation)
> > > of the "identity" of its containing directory and the file's own
> > > distinguishing name _within that directory_.  This is different
> > > compared to, say, bill-of-material structures or genealogical
> > > relationships, where the "nodes" (parts, people, ...) have a property
> > > "of their own" that uniquely identifies them "within the entire
> > > universe".  This is not the case for file systems.  *IX systems may
> > > have multiple directories each containing a /bin "file", and all
> > > those /bin "files" are distinct things.  Windows systems have multiple
> > > directories each containing an "Application data" folder, and those
> > > are all distinct.
>
> > > So if file systems are what's in your mind, then what you would
> > > typically need (for representing that in a database) is a single table
> > > where the _fully qualified_ filename is a key/identifier.  And as soon
> > > as you have that, there typically is no longer a need for explicitly
> > > recording the "identity" of the parent directory/folder, as that is
> > > already implied by the full name of the file itself.
>
> > > I don't fully see what you mean by "creating an entry in the
> > > directories table that points to a file".  Relational database designs
> > > do not include "pointers".  And you don't say on the directory level
> > > which files it contains, instead you say on the file level to which
> > > directory each file belongs.- Hide quoted text -
>
> > > - Show quoted text -
>
> > I think Frank's analogy is on all fours.
>
> > The nested sets model is consrained to those graphs which represent
> > hierarchies.  While each node will have a identity, users in Frank's
> > application will be working with descriptors, which is one level
> > removed from whatever identifier Frank sets up, and two levels removed
> > from the physucal structure of the database.  Leaf nodes in the
> > structure will correspond 1-1 to entries in a separte product table,
> > from which users will be trying to retrieve information.
>
> > The situation with a file system is very similar.  The directory
> > structure is a hierachical set of nodes.  While each node has an
> > identity in the logical disk structures. user of the file system work
> > with the descriptive names associated with each node, which are on the
> > order of 2 levels removed from the physical structure of the disk.
> > Leaf nodes in the structure correspond 1-1 to files which have a
> > different structure on the disk.
>
> > While the hierarchy represents a conceptual structure, that conceptual
> > structure does not exist in isolation.  Elements of it are already
> > implemetned in the product table.
>
> > I agree that there are differences as well as similarities.  That's
> > why I used the word "similar" and Frank used the word "like".
>
> > Fred.- Tekst uit oorspronkelijk bericht niet weergeven -
>
> > - Tekst uit oorspronkelijk bericht weergeven -
>
> I have no clue what point you're trying to make with your first
> paragraphs, but I just want to note that those very same "differences
> and similarities" are probably also the reason why I wrote "NOT THE
> BEST example [for dealing with graph data]".- Hide quoted text -
>
> - Show quoted text -

I'm starting to think some of the people on this topic can't read very well. Frank wasn't trying to create an example for dealing with graph data. He was trying to solve a business problem using nested sets to support hierachical lookups in his product data and was asking a specific question about how to organize the hierarch to do this. If you wish to complain that he was off topic for a theory group, I can't argue to the contrary. Still, I can't see that it hurts theory too much to apply it once in a while.

Fred. Received on Wed May 18 2011 - 08:27:23 CDT

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