1

Designing Semistructured Databases Using ORA-SS Data Model

ORA-SS: An Object-Relationship-Attribute Model for

Semistructured Data

2

Contents Part I. ORA-SS: An Object-Relationship-Attribute

Model for Semistructured Data – Object class – Relationship Type – Attribute – Semistructured data instance – Functional dependencies and other constraints – Inheritance hierarchy

Part II. Applications of ORA-SS Model – Normal form ORA-SS schema diagram – Storage schema for ORA-SS/XML databases – ORA-SS/XML views – Evaluating XML queries on ORA-SS databases – Translating relational schema into ORA-SS schema – Integration of XML documents

3

Motivation – An XML Document <department number =“cs”> <name> computer science</name> <course number = “cs4221”> <name> Database </name> <student number = “1234” > <name> B.Y.Smith</name> <grade> 70</grade> </student> <student number=“1235”> <name> C.U.Brown </name> <grade> 60 </grade> </student> … </course> … </department>

<! ELEMENT department (name,course+)> <! ATTLIST department number ID #REQUIRED> <! ELEMENT course (name, student*)> <! ATTLIST course number ID #REQUIRED> <! ELEMENT student (name, grade?)> <! ATTLIST student number CDATA #REQUIRED> <! ELEMENT name (#PCDATA)> <! ELEMENT grade (#PCDATA)>

(a) XML document

(b) An XML DTD for (a)

Notes: (1) Student’s number cannot be declared as ID. (2) The subelements name and grade are defined in the

same way. However, name is a property of student but grade is a relationship attribute.

4

Motivation (cont.)

(a) OEM Diagram Figure 1: Sample instance demonstrating OEM and Dataguide

♦ number

(b) Dataguide

▼♦ department

▼♦ student

▼ ♦ course ♦ name ♦ number

♦ name ♦ grade

♦ number ♦ name

department

cs

student

number name

computer science

&3

&1

course course

number

&20

name

&21 &22

number

grade

1234 B.Y.Smith 70 1235 C.U. Brown 60

cs4221 Database

&26

&7

student

&4

&2

name

&5

student

&6

number

&23

name

&24

grade

&25

number name

&27 &28

name of student and grade are defined in the same way. Bad!

5

Motivation (cont.)

Figure 3: ORA-SS schema diagram

number: CS4221

Student

Figure 2: ORA-SS instance diagram

name: Computer

Science course course

number: CS

department

name: Database

student

number: 1234

name: B.Y. Smith

grade: 70

student

number: 1235

name: C.U. Brown

grade: 60

number name

number

department

course 2, 1:n, 1:1

student number name

name grade

cs

number name CS, 2, *, +

Note: grade is declared as an attribute of the binary relationship type cs.

6

ORA-SS - The Data Model Object class

– attributes of object class – ordering on object class – weak object class

Relationship Type – attributes of relationship type – degree of n-ary relationship type – participation of objects in relationship type – disjunctive relationship type – recursive relationship type – IDD (identifier dependency) relationship type

Note that DTD and XML Schema have no concept of relationship type, they only describe the hierarchical structure of the xml documents.

Reference

7

ORA-SS

The Data Model (cont.)

Attribute – Attribute of object class or relationship type – key attribute (identifier) – composite attribute – disjunctive attribute – attributes with unknown structure – ordering on attribute – fixed and default values of attribute – derived attribute

Semistructured data instance

Functional dependencies and other constraints

Inheritance hierarchy

8

The data model of ORA-SS

Object Class

student

number first name last name

Figure 4: Object class student with attributes in an ORA-SS schema diagram

– represented by a rectangle

– attributes of object class by a circle

– identifier of object class by a filled circle

– ordering on object class by a symbol <

identifier Single valued

attribute

Object class

9

The data model of ORA-SS Relationship Type

– attributes of relationship type – degree of n-ary relationship type – participation of objects in relationship type – disjunctive relationship type – recursive relationship type

project

id name member

name job title publication

number title

2, +, +

2, *, +

(a) ORA-SS Schema Diagram

(b) Relationship instance

♦ number ▼♦ publication

▼♦ project

▼♦ member ♦ name

♦ title

♦ job title

♦ name ♦ id

(c) Dataguide

Figure 5: Representing binary relationship type

p1

p2

p3

m1

m2

pub1 pub2

pub3

project member publication

10

The data model of ORA-SS

Relationship Type (cont.)

project

id name member

name job title publication

number title

2, +, +

3, *, +

(a) ORA-SS Schema Diagram

Figure 6: Representing ternary relationship type

(b) Relationship instance

p1

p2

p3

m1

m2

pub1 pub2

pub3

project member publication

(c) Dataguide

♦ number ▼♦ publication

▼♦ project

▼♦ member ♦ name

♦ title

♦ job title

♦ name ♦ id

11

The data model of ORA-SS

IDD Relationship Type Some object classes may not have identifier. E.g. chapters of a book do not have a unique

identifier.

However, a chapter of a book can be identify by the identifier of the book (i.e. ISBN) and the chapter number of the chapter.

We call the object class chapter a weak object class with a weak identifier chapter_no.

The relationship type between book and chapter is called an identifier dependency (IDD) relationship type. Chapter is a dependent object class of the book object class.

It is similar to weak entity type and ID relationship type in ER approach.

IDD

book

chapter

section

ISBN title author

+

IDD

chapter_no

section_no

Figure: IDD relationship types

12

The data model of ORA-SS Attribute

– key attribute or Object Identifier (OID) – Attribute of object class and relationship type – composite attribute – disjunctive attribute (denoted by | ) – attribute with unknown structure (denoted by ANY)

– ordering on attribute (denoted by < )

– fixed and default values of attribute (denoted by F and D)

– derived attribute (denoted by dotted circle)

hobby

course

student * cno title ANY

dept prefix D: comp

number * sno first

name last name

mark grade

cs cs

cs, 2, 4:n, 3:8

Figure 7: Object classes with relationship type and attributes in an ORA-SS schema diagram

composite attribute

CS is a binary relationship type

Each course has min 4 students and no max.

mark is a CS relationship

attribute

Frist_name and last_name form a composite key

Each student min takes 3 courses and max 8.

13

The data model of ORA-SS Ordering

– the instances of an object class can be ordered, – the values of an attribute can be ordered, and – the set of attributes of an object class can be ordered

Figure 8: Ordered object classes and attributes in an ORA-SS schema diagram

book

isbn

chapter

number

IDD title

section

number

2,1:n, 1:1, <

3,1:n, 1:1, < IDD

author +

<

14

The data model of ORA-SS Disjunction

– disjunctive object class, and – disjunctive attribute.

Figure 9: Disjunctive attribute and relationship type in an ORA-SS schema diagram

cno

number street

course

title Exam_venue

lecture theatre laboratory student

sno name grade

hostel

cs, 2, 4:n, 3:8

address cs

house

15

The data model of ORA-SS References

Figure 10: Referencing an object class in an ORA-SS Schema Diagram

Student’

grade

cs, 2, 1:n, 1:m

cs

course

title cno

lecture theatre

laboratory

* text

book

student

sno address

first name

last name

name exam venue

sno

- Similar to IDREF in DTD

Note: name is a composite attribute

16

The data model of ORA-SS References (cont.)

Figure 11: Example of a recursive relationship type in an ORA-SS Schema Diagram

course

cno title prereq

2, 0:5, 1:n

prereq-cno

17

The data model of ORA-SS References (cont.)

Figure 12: Representing a binary relationship type as 2 symmetric relationship types to facilitate symmetric queries. Redundancy occurs.

cs

course

Student’

cno number of students

cs, 2, +, +

student

Course’

sno name

sc, 2, +, +

grade sno cno

sc

grade

18

The data model of ORA-SS Functional Dependencies

student, course → tutor tutor → course

1/n

n/1 n/- student course

tutor

sct

Figure 15: Functional dependencies diagram

19

The data model of ORA-SS Inheritance Hierarchy

(a) ORA-SS Schema Diagram

staff number

person

payrate degree

student

tutor

ssn

sno

faculty

(b) Inheritance Diagram

Figure 16: ORA-SS schema diagram and inheritance diagram

staff number sno

3,+,+ 3,+,+ grade title

sc, 2,+,+ sno

course

student

faculty tutor

ssn ssn payrate degree

ssn

cno sc

sno is another key

20

The data model of ORA-SS Constraints Inclusion dependency Semantic dependencies

(a) jdate of employee joined company, i.e. jdate is semantically determined by employee only.

(b) jdate of employee joined department, i.e. jdata is semantically determined by employee and department.

Figure 17: Modeling join date (jdate) ORA-SS schema diagram

ed, 2, 1:n, 1:1

jdate name ssn

department

employee

?

dno

ed

name 2,1:n,1:1

jdate name ssn

department

employee

?

dno name