Re: How to 'normalise' this scenario

From: Erwin <e.smout_at_myonline.be>
Date: Wed, 18 May 2011 04:14:48 -0700 (PDT)
Message-ID: <88b916d2-a232-416e-b41b-d4ae0161ff4a_at_y12g2000yqh.googlegroups.com>



On 17 mei, 21:50, "Fred." <ghrno-goo..._at_yahoo.com> wrote:
> 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:
>
> > > > On 16 mei, 08:50, Frank Millman <fr..._at_chagford.com> wrote:
>
> > > 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.
>
> > > 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]". Received on Wed May 18 2011 - 06:14:48 CDT

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