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Nov 7, 2005, 3:43:28 PM11/7/05

to

Hi!,

I am building a web directory similar to the dmoz and the yahoo

directory. The categories get updated often.

How do I find the subcategories from just the names of the ancester and

the current categories? For example, both the dmoz and yahoo

directories have filesystem-like URLs instead of category IDs. How do I

do that with nested sets or nested intervals? If not, how do they do

it?

There should be a count for how many listings in each subcategory. It

is much more efficient to count how many listings are in each

subcategory with nested sets than the adjacency list. Is there a way to

count subcategories efficently with nested intervals with farey

fractions?

Thank you very much!

Message has been deleted

Nov 7, 2005, 6:07:27 PM11/7/05

to

Subcategories can be counted "semi-efficiently", that is you have to

find all the intervals (or just all the left boundary points) that are

contained within the ancestor interval. The speed is proportional to

the size of the subtree.

Nested sets is often quoted as a solution for static hierarchies. It is

rarely mentioned that one can't query ancestors path efficiently.

Admittedly, nested sets counts the descendants nicely as rgt-lft-1, but

"count" is the only hierarchical total aggregate function that nested

sets can do efficiently.

The implementation of ancestor query in nested intervals encoding is

somewhat messy. If we ignore the efficiency, then

select count(1)

from tree t

where compare(rational(a,b), lft) > 0 and

compare(rgt(lft),rational(c,d)) >= 0

should be good enough. The [a/b, c/d] is the encoding of your node,

below which you count the descendants. This is full table scan of the

tree table. This could be improved by adding explicit predicate to turn

the access path to index range scan

select count(1)

from tree t

where compare(rational(a,b), lft) > 0 and

compare(rgt(lft),rational(c,d)) >= 0

and t.lft.num/t.lft.den between

a/b-0.00000001 and c/d+0.00000001

where the magic constant 0.00000001 is designed to offset floating

point numerical imprecision.

Nov 8, 2005, 3:03:13 PM11/8/05

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>> Is there a way to count subcategories efficently with nested intervals with farey fractions? <<

In the nested sets model, (rgt -lft +1 )/2 = size of subtree at this

root node.

Nov 8, 2005, 4:23:38 PM11/8/05

to

Sorry, please correct my interpretation of your problem, as it sounds

too simple to be true: you have a tree (of categories); given a node (a

category), you want to know its immediate children.

too simple to be true: you have a tree (of categories); given a node (a

category), you want to know its immediate children.

Nov 9, 2005, 6:06:48 AM11/9/05

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Oops... you want to count the descendants. Sorry, browser problem. Ok,

to count the descendants, I see you've already got good answers for SQL

systems. Personally I would use a non SQL system, but that's another

story.

to count the descendants, I see you've already got good answers for SQL

systems. Personally I would use a non SQL system, but that's another

story.

/* Namely the story of graph-based vs. relational systems. "Relational

atabases are universally conceived of as an advance over their

predecessors network and hierarchical models." (Vadim on dbazine.com)

This is probably true in the historical context of Ingres vs. Codasyl

etc. but mostly a myth. Unfortunately Codasyl's demise is (incorrectly)

perceived as a weakness of *all* graph-based data systems. (But why are

file systems still hierarchical today?) */

Nov 9, 2005, 7:31:53 AM11/9/05

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<amado...@netcabo.pt> wrote in message

news:1131534408.8...@z14g2000cwz.googlegroups.com...

> (But why are

> file systems still hierarchical today?) */

news:1131534408.8...@z14g2000cwz.googlegroups.com...

> (But why are

> file systems still hierarchical today?) */

Why indeed! It doesn't actually make a lot of sense. It is great for some

needs but by no means all. A more versatile organization which could be

viewed as a hierarchy (when that is useful) seems long past due.

Roy

Nov 9, 2005, 8:04:28 AM11/9/05

to

Is it another case of a "read a book or something" ? Codd's "A

Relational Model of Data for Large Shared Databanks" may be a good

start in order to understand why "graph-based systems" are not a good

approach to managing data.

Nov 9, 2005, 8:34:34 AM11/9/05

to

<<Is it another case of a "read a book or something" ?>>

No.

<<"graph-based systems" are not a good approach to managing data>>

Repeating a myth does not make it true. I'll gladly discuss any

technical issue.

Nov 9, 2005, 10:02:04 AM11/9/05

to

Calling Codd's work a myth is cute, but can you offer any technical

argument in favour of "graph-based systems" in addtion to what has been

rehashed on this newsgroup many times and besides "I say so" ?

Nov 9, 2005, 12:23:44 PM11/9/05

to

> Calling Codd's work a myth is cute, but can you offer any technical

> argument in favour of "graph-based systems" in addtion to what has been

> rehashed on this newsgroup many times and besides "I say so" ?

> argument in favour of "graph-based systems" in addtion to what has been

> rehashed on this newsgroup many times and besides "I say so" ?

I don't recall the arguments in Codd's original article of 1970 or so.

Maybe I'll take a relook next time I stop by the University library.

Anyway the following holds.

Some semantic domains are better modelled with graphs and trees than

with relations. The idioms for trees and graphs in SQL is extremely

convoluted to say the least (the word "pathetic" comes to mind). The

original problem in this thread is an concrete example.

Relational star people themselves recognized shortcomings of the

relational theory for semantic modelling. This was a major factor in

the creation of the Entity-Relationship model. And guess what: the E-R

is a graph-based model. (Not to mention UML class diagrams, but I know

what you'll say: these are for programs, not data.)

There's graph-based and graph-based. As you I also don't subscribe to

the socalled "hierarchical" and "network" models as they are often

presented in relational circles. They seem to be a heritage of Codasyl

and alike systems of the 1960's. Fortunately others kept researching

and have come up with much better graph-based models e.g. Resource

Description Framework and even XML.

Nov 9, 2005, 1:09:59 PM11/9/05

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amado...@netcabo.pt wrote:

> Some semantic domains are better modelled with graphs and trees than

> with relations. The idioms for trees and graphs in SQL is extremely

> convoluted to say the least (the word "pathetic" comes to mind). The

> original problem in this thread is an concrete example.

> Some semantic domains are better modelled with graphs and trees than

> with relations. The idioms for trees and graphs in SQL is extremely

> convoluted to say the least (the word "pathetic" comes to mind). The

> original problem in this thread is an concrete example.

This is because the domain is poorly understood. I would challenge the

idea that XML people have clearer understang of graph problems. Tags

don't magically solve problems.

> Relational star people themselves recognized shortcomings of the

> relational theory for semantic modelling. This was a major factor in

> the creation of the Entity-Relationship model. And guess what: the E-R

> is a graph-based model. (Not to mention UML class diagrams, but I know

> what you'll say: these are for programs, not data.)

Well, the join operation is another binary relation that can be

interpreted as edges in a graph. Excuse me, but that is not a

particularly deep idea. I fail to see it utilized anywhere except query

optimization.

> There's graph-based and graph-based. As you I also don't subscribe to

> the socalled "hierarchical" and "network" models as they are often

> presented in relational circles. They seem to be a heritage of Codasyl

> and alike systems of the 1960's. Fortunately others kept researching

> and have come up with much better graph-based models e.g. Resource

> Description Framework and even XML.

It is not a matter of "do hierarchical and network models make sence".

It's a matter of how nice is the underlying theory. So far relational

people seems to be unimpressed.

Nov 9, 2005, 1:20:30 PM11/9/05

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amado.al...@netcabo.pt wrote:

> > Calling Codd's work a myth is cute, but can you offer any technical

> > argument in favour of "graph-based systems" in addtion to what has been

> > rehashed on this newsgroup many times and besides "I say so" ?

>

> I don't recall the arguments in Codd's original article of 1970 or so.

> Maybe I'll take a relook next time I stop by the University library.

> Anyway the following holds.

>

> Some semantic domains are better modelled with graphs and trees than

> with relations.

Before going any farther, let's get our definitions straight in order

to prevent mutual misunderstanding. Please define the "semantic

domain".

>The idioms for trees and graphs in SQL is extremely

> convoluted to say the least (the word "pathetic" comes to mind).

The tools for treating tree-like structures in RDBs are pretty

straightforward, there is nothing "convoluted" or "pathetic" about

them. In addition to what has already been suggested, every major

database has some recursive query implementation that lets the OP solve

his problem easily.

>The

> original problem in this thread is an concrete example.

See above.

>

> Relational star people themselves recognized shortcomings of the

> relational theory for semantic modelling.

Who are "relational star people" ? Also, please define "semantic

modelling" .

> This was a major factor in

> the creation of the Entity-Relationship model. And guess what: the E-R

> is a graph-based model.

So what ? Chen's E-R model is *not* the relational model . E-R models

can be mapped to relational models or non-relational models.

>(Not to mention UML class diagrams, but I know

> what you'll say: these are for programs, not data.)

Let's not mention UML ;)

>

> There's graph-based and graph-based. As you I also don't subscribe to

> the socalled "hierarchical" and "network" models as they are often

> presented in relational circles.

If you do not subscribe to, say, the network model, then you can

probably spell out how the/(a) graph-based model is different from the

network model.

>They seem to be a heritage of Codasyl

> and alike systems of the 1960's. Fortunately others kept researching

> and have come up with much better graph-based models e.g. Resource

> Description Framework and even XML.

How is RDF or XML different from the network model ?

Nov 9, 2005, 4:01:46 PM11/9/05

to

Thank you very much, Vadim Tropashko.

Nested Intervals are very efficient for updating and selecting

hierarchical categories. I am creating a web application/directory from

it, but it would be more convenient for the user to retrieve its

children from an array of folder names in the URL (seperated by dots or

slashes, like google groups does for its usenet categories) instead of

a category_id.

Both dmoz and yahoo also retrieves its immediate subcategories by the

array of folder-names in the URL; Unlike some other directories that

retrives its immediate subcategories from a category_id in the URL. Is

there any efficient way to retrieve its immediate subcategories by an

array of folder-names such as dmoz and yahoo?

Basically, it's like replicating a file system: an array of folder

names can point to the category_id to retrieve their children. However,

it would take recursive SELECTs to find the category_id from the array

of folder names. I could store the whole path in each row, but it would

be very inefficient to update a category. Is there an efficient way to

do this?

Thank you very much!

Nov 9, 2005, 4:08:02 PM11/9/05

to

asdf wrote:

> Thank you very much, Vadim Tropashko.

>

> Nested Intervals are very efficient for updating and selecting

> hierarchical categories. I am creating a web application/directory from

> it, but it would be more convenient for the user to retrieve its

> children from an array of folder names in the URL (seperated by dots or

> slashes, like google groups does for its usenet categories) instead of

> a category_id.

>

> Both dmoz and yahoo also retrieves its immediate subcategories by the

> array of folder-names in the URL; Unlike some other directories that

> retrives its immediate subcategories from a category_id in the URL. Is

> there any efficient way to retrieve its immediate subcategories by an

> array of folder-names such as dmoz and yahoo?

>

> Basically, it's like replicating a file system: an array of folder

> names can point to the category_id to retrieve their children. However,

> it would take recursive SELECTs to find the category_id from the array

> of folder names.

So why don't you just use a simple adjacency list and a recursive query

instead ? What database are you using ?

Nov 9, 2005, 4:24:16 PM11/9/05

to

I use MySQL. The inefficiency for the adjacency list method grows

exponentially with the amount of categories.

exponentially with the amount of categories.

Nov 9, 2005, 4:42:47 PM11/9/05

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asdf wrote:

> I use MySQL. The inefficiency for the adjacency list method grows

> exponentially with the amount of categories.

It's unclear how one can achieve an exponential performance degradation

with an adjacency list traversal. Could you post the piece of code you

use to do that ?

There are quite a few tree traversal implementations for MS SQL Server

2000 on the Internet. You can google for "SQL Server" and "tree

traversal".

Nov 9, 2005, 5:24:03 PM11/9/05

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Mikito, the domains are not poorly understood, the join operation per

se does not give you a graph, the underlying theories are very nice

thank you, see e.g. RDF, XML, XQuery and XPath Data Model, and the

non-relational algebras XQuery Algebra, TAX, RAL etc.

se does not give you a graph, the underlying theories are very nice

thank you, see e.g. RDF, XML, XQuery and XPath Data Model, and the

non-relational algebras XQuery Algebra, TAX, RAL etc.

Nov 9, 2005, 5:54:49 PM11/9/05

to

Semantic domain is as you know the fancy name for the "things" you

represent with your schema and data.

represent with your schema and data.

Relational *key* people (sorry, "star" was bad choice of word, unless I

cited Kleene too:-) you know and cited two, Codd, Chen, and then

there's Date, Ulman, Wiederhold, etc. and I would not mind citing here

the people siting at the W3C working groups for XQuery, XPath, etc.

E-R maps better to graph-based representations than to relational ones.

It is not a matter of "can" but of "how well."

No I won't mention UML nor say the dirty word ("semi-structured":-)

There's lots of (good) graph-based models. RDF is (very) different from

XML and both are (very) different from Codasyl and my baby called

Mneson is (very, not so much, less) different from Codasyl, XML, RDF.

(I suggest Mneson be discuss in the thread "Implementing a graph

algebra" because that's it)

The "network" model in particular, or Codasyl: nodes are tables with

records and attributes. Not a pure graph. Actually not very graph-like

at all. I think the technical term is "a dirty mess."

How is RDF or XML different from Codasyl in particular: a lot

different. An RDF node is a single concept, not a table like in

Codasyl. An RDF edge is a single property out of a possibly (depending

on the meta-schema) open set of properties, not the single relation

"child" of Codasyl. Etc. RDF is a much cleaner model. A true graph

model. This has good consequences at many levels (theory, practice,

adoption). For example you can find very nice algebras for RDF out

there.

Nov 9, 2005, 7:00:22 PM11/9/05

to

I assume that you asked how to query immediate descendants (children!)

in the nested intervals model. I've emailed the chapter snippet on

nested interval encoding to your yahoo email address. Look up page 15

where it develops that nested intervals matrix encoding is an adjacency

model.

Nov 9, 2005, 7:30:04 PM11/9/05

to

I can't understand why no one has suggested Pick for this. Seems

perfectly simple to me. Anyone who has used Pick would not think twice

about this. The solution to the OP's problem would be trivial in the

extreme.

perfectly simple to me. Anyone who has used Pick would not think twice

about this. The solution to the OP's problem would be trivial in the

extreme.

Mike.

Nov 9, 2005, 8:19:10 PM11/9/05

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>> Nested sets is often quoted as a solution for static hierarchies. It is rarely mentioned that one can't query ancestors path efficiently. <<

An employee and all their Supervisors, no matter how deep the

organizational tree, with a column for sorting the path from root to a

node:

SELECT O2.emp, (O2.rgt- O2.lft) AS path_order

FROM OrgChart AS O1, OrgChart AS O2

WHERE O1.lft BETWEEN O2.lft AND O2.rgt

AND O1.emp = @myemployee

ORDER BY path_order DESC;

Nov 9, 2005, 8:22:17 PM11/9/05

to

<amado...@netcabo.pt> wrote in message

news:1131576889.9...@z14g2000cwz.googlegroups.com...

> Semantic domain is as you know the fancy name for the "things" you

> represent with your schema and data.

So what's the difference between a domain and a semantic domain ? If there

is none, why muddy waters with the fancy names like "semantic" ?

>

> Relational *key* people (sorry, "star" was bad choice of word, unless I

> cited Kleene too:-) you know and cited two, Codd, Chen, and then

> there's Date, Ulman, Wiederhold, etc. and I would not mind citing here

> the people siting at the W3C working groups for XQuery, XPath, etc.

>

> E-R maps better to graph-based representations than to relational ones.

> It is not a matter of "can" but of "how well."

>

> No I won't mention UML nor say the dirty word ("semi-structured":-)

>

> There's lots of (good) graph-based models. RDF is (very) different from

> XML and both are (very) different from Codasyl and my baby called

> Mneson is (very, not so much, less) different from Codasyl, XML, RDF.

> (I suggest Mneson be discuss in the thread "Implementing a graph

> algebra" because that's it)

Could you pleas give a definition of the graph algebra ?

>

> The "network" model in particular, or Codasyl: nodes are tables with

> records and attributes. Not a pure graph. Actually not very graph-like

> at all. I think the technical term is "a dirty mess."

>

> How is RDF or XML different from Codasyl in particular: a lot

> different. An RDF node is a single concept, not a table like in

> Codasyl. An RDF edge is a single property out of a possibly (depending

> on the meta-schema) open set of properties, not the single relation

> "child" of Codasyl. Etc. RDF is a much cleaner model. A true graph

> model. This has good consequences at many levels (theory, practice,

> adoption). For example you can find very nice algebras for RDF out

> there.

In order to be more specific, would you please elaborate on the RDF algebra.

like, give a definition of the algebra ?

>

Nov 9, 2005, 8:56:30 PM11/9/05

to

Joe,

We have already discussed this. The predicate

O1.lft BETWEEN O2.lft AND O2.rgt

looks simple, but is assymmetric regarding O1 and O2. In the acces path

driven from O2 to O1, the O1 set could be found by index range scan.

Find the first node in the b-tree satisfying

O1.lft BETWEEN O2.lft AND O2.rgt

then go to the next b-tree leaf, and so on. This is very efficient.

In contrast, the access path driven from O1 to O2 you have either to

scan the whole table O2, or use index range scan that would scan an

interval with one boundary condition only

O1.lft >= O2.lft

or

O1.lft <= O2.rgt

which would scan large portion of the index. This is (relatively)

inefficient.

Nov 9, 2005, 9:11:48 PM11/9/05

to

amado...@netcabo.pt wrote:

> Mikito, ..., the join operation per

> se does not give you a graph, ...

> Mikito, ..., the join operation per

> se does not give you a graph, ...

Take a set of all tables as a set of vertices. Take a set of all

foreign key integrity constraints as a set of edges. Do you challenge

the idea that I just defined a graph?

Nov 10, 2005, 4:14:48 AM11/10/05

to

Mikito Harakiri schrieb:

Yes, but in such a formulation it is not RM! It is a graph model

implemented via RM.

Nobody argues that many things can be implemented via RM including

graph-based models. I could even imagine that someone could implement

OOP or AOP techniques using RM. Or, vice versa, if I implement a

behaviour of RM using Turing machine then it does not mean that Turing

has invented RM.

The question is (as far as I understand it):

Is it more appropriate to view the (data) world as a set of relations

(with some operations, constraints etc.) or as a graph?

Nov 10, 2005, 6:57:07 AM11/10/05

to

<<In order to be more specific, would you please elaborate on the RDF

algebra.

like, give a definition of the algebra ?>>

algebra.

like, give a definition of the algebra ?>>

An RDF algebra is given in "RAL: an Algebra for Querying RDF" by

Fransicar et al. There are others. I remember seing very nice

discussions in the proceedings of the WWW conference some years ago.

For XML see: "TAX: A Tree Algebra for XML" by Jagadish et al. and

"XQuery 1.0 and XPath 2.0 Formal Semantics" by the W3C, which

supersedes "The XML Query Algebra."

For my own efforts see thread "Implementing a graph algebra."

Please note that I do not particulary favour RDF. I am interested in

more general languages, ideally one capable of solving the wild

disparity of Semantic Web languages found today. Maybe a bit like what

Codd did in the 1970's w.r.t. the disparity of database models.

Nov 10, 2005, 8:56:38 AM11/10/05

to

amado.al...@netcabo.pt wrote:

> <<In order to be more specific, would you please elaborate on the RDF

> algebra.

> like, give a definition of the algebra ?>>

>

> An RDF algebra is given in "RAL: an Algebra for Querying RDF" by

> Fransicar et al. There are others. I remember seing very nice

> discussions in the proceedings of the WWW conference some years ago.

>

No, that won't do. Please give your own RDF algebra definition, not

necessarily formal. Surely it cannot be that complicated.

> For XML see: "TAX: A Tree Algebra for XML" by Jagadish et al. and

> "XQuery 1.0 and XPath 2.0 Formal Semantics" by the W3C, which

> supersedes "The XML Query Algebra."

>

> For my own efforts see thread "Implementing a graph algebra."

You did not provide any definition of the algebra. Could you please do

it now ?

>

> Please note that I do not particulary favour RDF. I am interested in

> more general languages, ideally one capable of solving the wild

> disparity of Semantic Web languages found today.

I do not understand what you mean.

> Maybe a bit like what

> Codd did in the 1970's w.r.t. the disparity of database models.

Also, you did not clarify what you've meant by a "semantic domain" as

opposed to just a "domain". Could you offer a clarification ?

Nov 10, 2005, 11:56:31 AM11/10/05

to

Alexandr Savinov wrote:

> Mikito Harakiri schrieb:

> > amado...@netcabo.pt wrote:

> >

> > Take a set of all tables as a set of vertices. Take a set of all

> > foreign key integrity constraints as a set of edges. Do you challenge

> > the idea that I just defined a graph?

>

> Yes, but in such a formulation it is not RM! It is a graph model

> implemented via RM.

> Mikito Harakiri schrieb:

> > amado...@netcabo.pt wrote:

> >

> > Take a set of all tables as a set of vertices. Take a set of all

> > foreign key integrity constraints as a set of edges. Do you challenge

> > the idea that I just defined a graph?

>

> Yes, but in such a formulation it is not RM! It is a graph model

> implemented via RM.

I was merely suggesting that there is no more substance to the

Entity-Relationship model than in the above graph definition.

Nov 10, 2005, 12:59:50 PM11/10/05

to

I'll not give an algebra for RDF here since RAL is readily available

and I have my own algebra to worry about. So I'll give you a sketch of

the latter instead.

and I have my own algebra to worry about. So I'll give you a sketch of

the latter instead.

Mneson is a graph-based metamodel of data, based on a graph with the

following properties:

1. Connections are binary, directed, and untyped i.e. unlabelled. Note

that being untyped entails not having an explicit identifier. A

connection is identified solely by the (ordered) pair of vertices it

represents. Hence, there are no duplicate connections. That is, for any

pair of vertices (x, y) there can be at most one connection from x to

y. For a connection from x to y we say that x is a "source" of y and y

is a "target" of x.

2. A vertex is either a datum or a pivot.

(a) A data vertex or datum represents an atomic value of data e.g. an

integer, a real number, a symbol. The value of a data vertex is the

identity of the vertex.

(b) A pivot has no intrinsic value but it has nevertheless a unique

identity. In practice a serial number is used to identify pivot

vertices.

3. There are no island vertices. There may be, however, island

subgraphs.

A database instance is expressed in the base structure by means of

special subgraph topologies that represent complex data entities like

attributes, lists, sets, etc. [1] A possible definition is as follows:

Attribute. An attribute of name x1 and value x2 is represented by an

attribute instance x3 such that x1 connects to x3 and x3 connects to

x2. The attribute is connected to its subject x0 by connecting x0 to

x3. For example suppose that x5 represents an employee; the fact that

x5 has Social Security Number, or SSN (the attribute name) 123456789

(the attribute value) is expressed by connecting: x5 to x6, "SSN" to

x6, x6 to 123456789.

Set. For a vertex x that represents a set, the targets of x represent

the members of the set. In parlance we say x is the 'head' of the set.

Note this is a different object than the algebraic type set of vertices

below.

Etc.

The topologies may overlap, e.g. a vertex x that is an attribute type

may also be the member of a set, or even a set itself in which case the

respective attribute instance is a member of x.

An algebra (called the Mneson Calculus) is defined on the base graph as

follows. The algebraic type is the set of vertices. The operations

include the set operations (intersection, union, etc.) and the (unary)

adjacency operators T, S for Targets, Sources, with the obvious

semantics:

T (X) = {y : y is a target of x, x in X}

S (X) = {y : y is a source of x, x in X}

The Mneson Calculus serves as a query language. For example, the

following query retrieves the employee with SSN 123456789:

T ({Employees}) ^ S [T ({"SSN"}) ^ S({123456789})]

where ^ means set intersection.

Singleton arguments are very common so we take the liberty of ommiting

the {}, for clarity; and we want to focus on algebraic issues so often

we also ommit the "" around literals. With these simplifications the

expression above becomes

T (Employees) ^ S [T (SSN) ^ S (123456789)]

This example of course assumes a schema where vertex Employees is the

head of the set of employees.

________

[1] These entities are also called "semantic" structures because they

are designed to model semantic domains. Note how the semantic

structures are in turn modelled in terms of the base untyped graph. So

formally we have a chain of domains here and we say "semantic domain"

do describe the former level. Sometimes I say "semantic domain" even

when it is not necessary to distinguish, for habit I guess, I am sorry

if I sound pedantic, please just ignore the word on those occasions.

Nov 10, 2005, 3:30:57 PM11/10/05

to

Do you mean to say that the entity is presented as a graph whose nodes

are values of some type and edges are directed paths with two paths (S

and T) per each node ? If so, how is it different or better that the

traditional network model ?

Nov 10, 2005, 5:07:53 PM11/10/05

to

"Do you mean to say that the entity is presented as a graph whose nodes

are values of some type and edges are directed paths with two paths (S

and T) per each node?"

No. A node is either a datum or a pivot. An edge is directed. S, T

means the sources, targets of a node, if any. A node may have any

number of M sources, any number N of targets, as long as M + N > 0.

For a connection from x to y we say x is a source of y, y is a target

of x.

S, T are also called A+, A- in the literature (A from "Adjacent").

The "traditional" network model being Codasyl, nodes are tables, and

edges have a predefined semantic value, member I think, I don't have

the reference at hand, anyway very different (and anyway I'm even less

interested in Codasyl than in RDF).

Nov 10, 2005, 5:25:31 PM11/10/05

to

<amado...@netcabo.pt> wrote in message

news:1131660473.5...@g43g2000cwa.googlegroups.com...

> "Do you mean to say that the entity is presented as a graph whose nodes

>

> are values of some type and edges are directed paths with two paths (S

> and T) per each node?"

>

> No. A node is either a datum or a pivot. An edge is directed. S, T

> means the sources, targets of a node, if any. A node may have any

> number of M sources, any number N of targets, as long as M + N > 0.

So how would you represent , say, three entities like 'employee',

'department', 'company' ?

Nov 10, 2005, 7:07:56 PM11/10/05

to

Each single concept is represented by a vertex. The properties of the

concept are represented by topologies involving the vertex. The fact

that employee x10 works in the accounting department may be modelled as

concept are represented by topologies involving the vertex. The fact

that employee x10 works in the accounting department may be modelled as

Employees => x10

x10 => x11

WorksIn => x11

x11 => Accounting

where x => y means connected from x to y. Note that this model makes

similarly possible and simple to query for where does x10 work

T (T (x10) ^ T (WorksIn))

and for all workers in accounting

T (Employees) ^ S [S (Accounting) ^ T (WorksIn)]

(A more demanding schema would require a department be also a pivot, so

it could have attributes attached, and be a member of a Departments

like employee x10 is a member of Employees.)

Nov 10, 2005, 7:39:36 PM11/10/05

to

amado...@netcabo.pt wrote:

> Each single concept is represented by a vertex. The properties of the

> concept are represented by topologies involving the vertex. The fact

> that employee x10 works in the accounting department may be modelled as

>

> Employees => x10

> x10 => x11

> WorksIn => x11

> x11 => Accounting

>

> where x => y means connected from x to y.

> Each single concept is represented by a vertex. The properties of the

> concept are represented by topologies involving the vertex. The fact

> that employee x10 works in the accounting department may be modelled as

>

> Employees => x10

> x10 => x11

> WorksIn => x11

> x11 => Accounting

>

> where x => y means connected from x to y.

Before going to queries, could you please work out, that is demontrate

what the graph is for the following two relations

Employees

Name Dept Salary

------ ------ -------

Smith 1 1000

Scott 2 2000

King 2 3000

Departments

Deptno Name

------ ------

1 Accounting

2 Manufacturing

Nov 10, 2005, 8:10:19 PM11/10/05

to

In the Dot language (Graphviz from AT&T):

digraph {

Employees -> x1

Employees -> x2

Employees -> x3

Name -> x4

Name -> x5

Name -> x6

Dept -> x7

Dept -> x8

Dept -> x9

Salary -> x10

Salary -> x11

Salary -> x12

Departments -> x13

Departments -> x14

Deptno -> x15

Deptno -> x16

Name -> x17

Name -> x18

x1 -> x4 -> Smith

x1 -> x7 -> x13

x1 -> x10 -> 1000

x2 -> x5 -> Scott

x2 -> x8 -> x14

x2 -> x11 -> 2000

x3 -> x6 -> King

x3 -> x9 -> x14

x3 -> x12 -> 3000

x13 -> x15 -> 1

x13 -> x17 -> Accounting

x14 -> x16 -> 2

x14 -> x18 -> Manufacturing

}

Note this is a canonical, literal translation of the given relational

database. Modelling the semantic domain directly on Mneson would

simplify things. For example the Deptno is probably not necessary

semantically but just a requirement of the relational model for a key,

or for the connection (join), or both. Mneson does not need keys. A

distinct entity is a distinct vertex. Connected entities simply share a

topology.

Nov 10, 2005, 8:21:21 PM11/10/05

to

<amado...@netcabo.pt> wrote in message

news:1131667676....@g47g2000cwa.googlegroups.com...

> Each single concept is represented by a vertex. The properties of the

> concept are represented by topologies involving the vertex. The fact

> that employee x10 works in the accounting department may be modelled as

>

> Employees => x10

> x10 => x11

> WorksIn => x11

> x11 => Accounting

>

> where x => y means connected from x to y. Note that this model makes

> similarly possible and simple to query for where does x10 work

>

> T (T (x10) ^ T (WorksIn))

>

> and for all workers in accounting

>

> T (Employees) ^ S [S (Accounting) ^ T (WorksIn)]

Ok, I see. Now, please explain the following:

1. How is the graph model navigation different from the old Codasyl network

model pointer chasing, as what you apparently do is:

a. go one step down from the Employee node and collect all its children

b. go one step up from the Accounting node and collect all its "parents"

c. go one step down from WorkIn and collect all its children

d. find the intersection of sets from step (b) and (c)

e. go one step up for each element in the set constructed at step (d)

f. find the intersection of sets constructed at step (a) and (e)

The only novelty in comparison to the old network model appears to be the

ability to use sets of nodes and set operations on them (which is not quite

a novelty since as far as I remember OMG's OQL could do the same).

2. The structure you've described is not an algebra because it's not closed

over the operations you've defined (compare to the relational algebra).

3. It's unclear how you handle attributes like age or salary (find all the

employees in accounts whose age is between 30 and 40 and salary is less than

40,000).

Nov 10, 2005, 8:42:38 PM11/10/05