1 ODMG: ODL and OQL This lecture is a short version of: Lecture 11 (on ODL) & Lecture 12 (on OQL)...

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ODMG: ODL and OQL

This lecture is a short version of:Lecture 11 (on ODL) &Lecture 12 (on OQL)

M. Akhtar AliSchool of Informatics

Advanced Databases (CM036) – Lecture # 11 & 12: The ODMG Standard for Object Databases 2

Lecture outline Different ways of representing links between objects Mapping Associations in ODL Different kinds of inheritance Handling inheritance in ODL OQL (object query language)


Different ways of representing links Single valued attribute

Single reference

Collection valued attribute

Collection of references

A B

A B

A B1, B2, … Bn

A B1B2

Bn


UNN-IS COMPLETECLASS DIAGRAM


Bi-Directional Associations in ODL

Implementing the to-one (1..1) relationship from Student to Course class.

relationship Course enrolledOn inverse Course::students;


Bi-Directional Associations in ODL …

Implementing the to-many (0..*) relationship from Course to Student class.

relationship set<Student> studentsinverse Student::enrolledOn;


Aggregation in ODL Week Aggregation (part-whole relationship)

Maps onto an attribute in the aggregating class. Single valued (if 0..1 or 1..1). Collection valued (if 0..* or 1..*). For example, a Course is a whole object made up of several

Module objects. The modules aggregation in the Course class is defined as

follows:

attribute set<Module> modules;

The application need to maintain the integrity of modules attribute and to make sure that the set must contain at least one reference to a module object.


Aggregation in ODL … Strong Aggregation (part-whole relationship)

Maps onto an attribute in the aggregating class. Single valued (if 0..1 or 1..1). Collection valued (if 0..* or 1..*). The aggregated class becomes a structured literal if it does not participate in

any association with another class. Otherwise, it becomes a self-standing class.

For example, a Person object owns at least one or more Passport objects. The aggregated class: Passport becomes a structure:

struct Passport {string passportNum;string nationality;date issueDate;date expiryDate; };

The passports aggregation in the Person class is defined as follows:attribute set<Passport> passports;

There is no automatic referential integrity for the passports attribute.


Association Classes When the association is one-to-one

The association can be implemented as bi- or uni-directional depending on the situation.

Add the attributes of the Association class to the participating class whose objects are more likely to get involved in the relationship.

For example, suppose that every department has a chair who is one of the lecturer’s in the department.

It would be better to define the association as an attribute (uni-directional relationship) in Department including startDate as only few lecturer will ever participate in the relationship.

attribute Lecturer hasChair;attribute date startDate;


Association Classes … When the association is one-to-many

The association can be implemented as bi- or uni-directional depending on the situation though preferably bi-directional.

Add the attributes of the Association class to the participating class on the to-many side.

For example, part-time lecturers works for a department for certain hours.

It is better to use bi-directional relationship for the above.

class PartTimeLecturer { ... relationship Department worksFor inverse Department::partTimeStaff; attribute unsigned short hours; };


Association Classes … When the association is many-to-many

The association class becomes a self-standing ODL class with bi-directional relationships to both participating classes.

The attributes of the association class becomes attribute of the ODL class representing it.

For example, the many-to-many association between Student and Module (having the association class) becomes:

class StudentModule (extent AllStudentMoudles) {attribute unsigned short marks;relationship Student ofStudent inverse Student::takes;relationship Module forModule inverse Module::takenBy; };


Association Classes … When the association is many-to-many …

The Student class will include to-many relationship with StudentModule class:

relationship list<StudentModule> takes inverse StudentModule::ofStudent;

list is used to keep track of the order in which modules were taken.

The Module class will include to-many relationship with StudentModule class:

relationship set<StudentModule> takenBy inverse StudentModule::forModule;


Association Classes … But in some situations you may implement many-to-

many Association classes differently. That is you may embed the association class inside one of the

participating classes if navigation from both sides is not desirable/required.

For example:

In the above case the association class can be embedded inside the Invoice class if it is not necessary for products to have information about invoices on which they appear.


Association Classes …struct ItemType {

unsigned short units;Product item;};

class Invoice (...) {... // attributes etcattribute set<ItemType> items;};

class Product (...) {... // attributes etc, no need to define attribute // of type Invoice or ItemType or relationship // with Invoice class.};

Invoice objects will not be dependent upon Product objects or will not require joins or navigation to obtain information about items.


Different kinds of inheritance Single verses Multiple Inheritance

In Single inheritance, a sub-class has only one super-class. In Multiple inheritance, a sub-class has more than one super-class.

Inheritance of Behaviour A sub-class inherits only the behaviour (operations) of a super-class.

The super-class in this case usually only defines operations i.e., an interface. For example, defining a common interface for students and lecturers.

It is also known as is-a or is-a-kind relationship. Inheritance of State

When a sub-class extends a super-class by adding attributes, operations or relationships and both of them are concrete classes. The sub-class inherits every thing defined in the super-class. For example, Lecturer and Student classes inherit all the attributes and operations of Person class.


Interface verses Class An interface is a classifier that represents an abstract

collection of operations.

A class may have one or more interfaces, and an interfaces can group operations of several different classes.

<<interface>>Identification

getAge(): integerchangeAddress(addr:Address)getBirthDate(): dategetName(): stringgetGender(): stringgetAddresses():set(Address)

Although Person class is super-class of Student and Lecturer, yet it would be useful to define a common interface.

Given the Identification interface, we can have several classes that would realize the same interface.


Example Class Diagram for UNN-IS


Handling inheritance in ODL The inheritance of behaviour is represented by “:”

For example:

class Person : Identification {...}; Defines that Person class inherits all the operations defined in the

Identification interface. The object type after the : must be an interface.

The inheritance of state is represented by “extends” For example:

class Student extends Person {...}; Defines that Student class inherits all the operations, relationships

and attributes defined in the Person class. A class extends another class, an interface may inherit from

another interface but not from a class.


Implementing Inheritance in UNN-IS

interface Identification { short getAge(); void changeAddress(in

Address addr); date getBirthDate(); string getName(); string getGender(); set<Address> getAddresses();};

class Person : Identification {...};

class Student extends Person {...};

class Lecturer extends Person {...};

class UGStudent extends Student {...};

class PGStudent extends Student {...};

class FullTimeLecturer extends Lecturer {...};

class PartTimeLecturer

extends Lecturer {...};


OQL – for querying the database OQL is the the query language in ODMG standard. OQL is a superset of the SQL (in select-from-where

clauses). It can be interactive as well as embedded in other

programming languages. It is a declarative language. OQL can invoke operations written in programming

languages (C++, Java, Smalltalk). The operators in the language can be freely

composed. OQL includes operators for creating collections,

arithmetic and logical operators, aggregation, sorting and grouping.


Retrieving Objectsselect l from l in Lecturerswhere l.address = “Newcastle”

or

select l from Lecturers as lwhere l.address = “Newcastle”

or

select l from Lecturers lwhere l.address = “Newcastle”

• The query returns all Lecturer objects who live in Newcastle. The type of the query result is bag<Person>.

• Whenever the scope of an OQL

query is a collection (e.g. Lecturers, a set of Lecturer objects), we define an iterator variable that ranges over the collection (e.g. l in Lecturer). In the query l denotes an object of type Lecturer.


Database Entry Points Named objects are entry points to the database (same

as table or view names in RDBs). Class extents are usually used to refer to in OQL

queries. Some frequently used objects can be assigned unique

names e.g. ComMath a named Department object or AdvDB as named Unit object.

To use named objects, you don’t have to write a query in select-from-where form. You can simply write:

ComMath.staff which returns all staff member objects in the school of

computing and mathematics.Students

which returns a set of all Student objects in the database.


Retrieving data from multiple objects

Without using Joinsselect struct(LName:l.name, DName:l.worksFor.name)from l in Lecturers The query contains an implicit join based on the relationship

that exists between Lecturer and Department object types and due to the path expression l.worksFor.name. However, the user does not need to tell the OQL compiler.

Using Joinsselect struct(LName:l.name, DName:d.name)from l in Lecturers, d in Departmentswhere l.worksFor = d Here the user explicitly tells that a join should be performed

on the basis of matching OIDs (l.worksFor and d denote an OID of a Department object).


Unnesting and Nesting Collections

Unnesting a collectionselect struct(LName:l.name, SName:s.name)from l in Lecturers, s in l.tutees The query retrieves names of lecturers and their tutees in pairs. The query iterates over the collection Lecturers via the variable l; then

iterates over l.tutees (a set of Student objects) via the variable s; then projects out name from l and name from s. The type of the query result is bag<LName:string, SName:string>.

Nesting a collectionselect struct(LName:l.name,

SNames:(select s.name from s in l.tutees)from l in Lecturers The query retrieves for every lecturer, his/her name and all his/her tutees

names. Here instead of unnesting l.tutees, the query iterates over l.tutees via s and

projects out name attribute and then constructs a collection out of it. The type of the query is bag<LName:string, SNames:bag<string>>.


Summary of OODB• An overview of ODMG standard

• An introduction to the Object Data Model

• Comprehensive introduction to ODL

• Introduction to OQL

1 ODMG: ODL and OQL This lecture is a short version of: Lecture 11 (on ODL) & Lecture 12 (on OQL)...

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