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Object-Role Modelling

Conceptual Schema Design

NIAM History

prior to 1975 - CODASYL Heirarchical DB model based loosely on relational model of data until Ted Codd left group and formalised relational model operations
1976 - Nijssen
1978 - Sub-types added, with NIAM evolving into knowledge representational technique as opposed to a database tool
1982 - ISO formally recognised Conceptual Schema Diagrams as valid, standard, method of knowledge representation
1982 - Van Bekkum published works on 'NIAM'
1983 - Wintraecken published 'NIAM bible'
1985 - Halpin and Nijssen published NIAM text
1995 - Halpin 2nd Edition of text


ORM - Examples to Elementary Facts

  • This is the most important (and difficult) step, and one of few that really require a person.
  • Consider it a 'fact-finding' mission (or rather 'fact-type finding' mission)
  • We search for examples of written communication (eg. printouts, data entry forms, graphs, tables, charts... used by user - these provide the 'raw' material to build an IS.
  • Our aim is to express sentences (in unambiguous formal English) in elementary form (NOT as the conjunction of simpler facts). Facts expressed in this form are said to be single-valied or atomic

Some terminology:

  • ENTITY = basic objects in UoD (these can be either physical/tangible or abstract)
  • ENTITY TYPE = the type of object, each example entity is an instance of one
  • ROLE = every entity plays at least one role. A role is played by at most one entity. All entities belong to at least one logical predicate.
  • UNARY FACT = predicate(entity) eg: Olivia is_male; or is_male('Olivia')
  • BINARY FACT = predicate(entity,entity) eg: Olivia has_gender Male; or has_gender('Olivia','male')
  • TERNARY FACT = predicate(entity,entity,entity) eg: Olivia obtained VHA in IPT; or obtained_mark_for('Olivia','IPT','VHA')
  • ARITY = the number of entities involved in a fact type

In our understanding of facts, context is important

Sydney is_located_in Wales (possible ambiguity?)

Q: Did you hear about the man with the wooden leg called Smith?
A: No, what did he call the other one?

label 'Smith' incorrectly applied to the entity 'Wooden Leg', rather than the correct entity (a person)

Elementary Sentences follow a syntax similar to the following:
Entity(ref)'data'|data predicate [Entity(ref)'data'|data ]
{predicate Entity(ref)'data'|data }


Examples

A: Parents and Progeny

Parents               Children
--------------------------------------
Ann, Bill             Colin, David, Eve
Boris, Carla          Olivia, Zac


Person(firstname)'Ann' isparentof person(firstname)'Colin'


B: Love Thy Neighbor

Country           Ally            Enemy
--------------------------------------------
Yugoslavia        Russia
USA               Australia       Iraq
Australia         USA             New Zealand
                  CUBA
CUBA              Russia          USA
                  Yugoslavia


Country(countryname)'Yugoslavia' hasally   Country(countryname)'Russia'
Country(countryname)'USA'        hasenemy  Country(countryname)'Iraq'



C: It Stunts Your Growth

Smokers        Nonsmokers
------------------------------
Fred           Eugene
Noreen         Beryl
Jim-Bob        Neville


unary:  Person(firstname)'Fred'   issmoker
        Person(firstname)'Eugene' isnonsmoker

binary: Person(firstname)'Fred' has Smokingstatus(sscode)'S'
        Person(firstname)'Eugene' has Smokingstatus(sscode)'N'


D: Meetings Schedule

Club       Day        Hour            Room
---------------------------------------------------
IPTSIG     Wed        7.30 a.m        4.8
Computer   Tue        3.15 p.m        4.9
Chess      Fri        3.30 p.m        4.6


Club(clubname)'IPTSIG' meetson Day(daycode)'Wed'
Club(clubname)'IPTSIG' meetsat Time(ampmcode)'7.30 a.m'
Club(clubname)'IPTSIG' meetsin Room(roomnum)4.8


E: First Year Offerings

Subject    CredPts    Semester        Enrollment      Lecturer
----------------------------------------------------------------------------
CS100          8          1               500             PP
CS102          8          2               500             GR
CS112          8          1               300             TH
CS380          16         2               45              AL

assuming a subject can run in more than one semester:
Subject(subjcode)'CS100' isofferedin Semester(semnum)1 = offering Subject(subjcode)'CS100' isworth Credit(points)8 offering istaughtby Lecturer(initials)'PP' offering hasenrollmentof Personcount(nr) F: Supermarket Ripoff Item Itemcode CostPrice Markup SellingPrice ---------------------------------------------------------------------------- Kittycat200g A0350 0.35 0.44 0.79 Selsun250ml C1104 2.40 0.85 3.25 Twistee100g X0042 0.10 0.69 0.79 Product(code)'A0350' is labelled Itemname 'Kittycat200g' Product(code)'A0350' wholesalesfor Money($)0.35 Product(code)'A0350' retailsfor Money($)0.79 Product(code)'A0350' hasmarkup Money($)0.44 G: Pets Animal Name Sex ----------------------------------- cat Flossie F dog Nip M iguana Leonard M Animal(name)'Leonard' is of kind Animal(animalname)'Iguana' Animal(name)'Leonard' has Gender(gendercode)'M' H: Co-Curricular Involvement Club Time Room Student Name Year Tut Group ----------------------------------------------------------------------- Chess Mon 3 pm 4.5 Pizza C 6 T2 Dingo S 12 W9 Parrts O 11 B2 Debat'n Wed 4.30 pm 5.4 Nurke F 12 T2 Parrts O 11 B2 Club(clubname)'Chess' meetsat Time(ampmcode) 3.00p.m. Club(clubname) isheldin Room(roomcode)'4.5' Student(studentname) 'Pizza C' belongsto Club(clubname) Student(studentname) isin Year(yearnum) 6 Student(studentname) ismemberof Tutgroup(tutgrpcode) T2 I: STUDENT RESULTS Student Subject Rating ------------------------------------------------ Bloggs F IPT VLA Nurke J " VLA Paarts O HEC HA : : : Student(studentname)'Bloggs F' received Rating(ratingcode)'VLA' in Subject(subjcode)'IPT' J: MANIACS ON THE ROAD Person Licence# Cars Driven ----------------------------------------------- Pie QT A3050 235PZN Adams G A2245 235PZN, 108AAQ : : : Person(personname)'Pie QT' holds Licence(licensenumber)'A3050' Person(personname)'Pie Qt' drives Car(regnum)'235PZN' K: SOFTWARE SELLERS Retailer Item Quantity Sold ------------------------------------------------------------ CompuWare SQL+ 330 ZappoPascal 330 WorPerfect6 200 SoftwareLand SQL+ 330 ZappoPascal 251 : : Retailer(retailername)'CompuWare' sold Item(itemname)'SQL+' inquantity Tally(number)330

ORM - First Draft of the CS

using symbols to represent entities and roles, draw each of the sentences as separate conceptual schema fragments. (to ensure they are correctly drawn, identify the roles and their converse roles, and attempt to instantiate the fact types)

a simple binary fact type

Notice, E1 and E2 represent two distinct objects (entities) in the UoD, r is the relationship (role) linking E1 to E2. r' is the role linking E2 to E1 (which in fact is the converse of r)

IMPORTANT - each role is played by exactly ONE entity, and an entity must play AT LEAST ONE role. If this is not the case, we have nothing tangible to store about the entities

'Purists' argue that there is worth in drawing individual schema fragments for each of the fact types represented in step 1 - I believe that most students are able to see that entities are shared amongst sentences and can often go straight to the first draft of the diagram

eliminating common entities

conversely, go straight to step 3


ORM - Eliminate Surplus Entities and Identify Derived Facts

This step requires you to examine each of the entities drawn in your schema, and COLLAPSE any common (called surplus) entities. Also, indicate that fact types are derived - place an asterisk beside the fact type if it is derived. This will later indicate that this fact type is not to be stored, rather it will resolve into a RULE.

the music database

The first three steps are designed to identify the types of objects that exist in the universe to be modelled, and the relational 'glue' that binds them together. The next few steps are designed to specify what is possible and impossible (ie. to constrain the objects with 'real-life' rules, so they are described as behaving in our model UoD just as they do in the wild.


ORM - Uniqueness Constraints

WARNING: using examples to deduce constraints requires knowledge of the UoD, or at least a little common sense. By specifying a real-life too weakly or too strongly we adversely affect the performance of our model. When referring to examples, they will be significant - should any inferences be made that are not directly supported by the examples, the underlying assumptions WILL BE WRITTEN DOWN.

Place uniqueness constraints ON EVERY FACT TYPE in the schema - be systematic, and check all combinations.

A Uniqueness constraint prohibits something from happening more than once. Uniqueness constraints exist in many combinations:

relationship types

The 'whole row' constraint is always considered to be present - this is because NO FACT CAN BE REPEATED - it is considered a weak constraint. If other uniqueness constraint patterns are found, it is often left out (as redundant)

Facts can appear in many forms. The 'nested' forms of the facts below say something different about the populations than do the 'flattened' forms.

flattened ternary
nested ternary

With the 'nested' forms, the nest object (enrollment) exists in it's own right, and may have a collection of 'attributes' that can be added LATER - the flattened form is an 'all or nothing' fact type - all three entities must be present before the fact can be stored.

flattened ternary constrained
nested ternary constrained

INTER-FACT-TYPE constraints are relatively common, and help us to define unique columns, secondary and foreign keys

inter-fact constraint

Given the following significant table populations, indicate appropriate uniqueness constraints from those illustrated:
which key?

    A    B     C           A    B    C            A    B    C
    a1   b1   c1           a1   b1   c1           a1   b1   c1
    a2   b1   c1           a2   b2   c2           a1   b1   c2
    a1   b2   c2           a1   b2   c2           a2   b1   c2
                           a2   b2   c1           a2   b2   c2
a more complex constraint

It is important to investigate each combination of entity when placing uniqueness constraints - these will translate to KEYS and INTER- TABLE CONSTRAINTS and are therefore vital to maintain the integrity of the system.

Also, correct placement of uniqueness constraints can help determine if the arity of the fact types drawn is correct.

Beware also of inconsistent constraints

illegal ternaries
equivalent forms

 

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