1 Lecture 10: Database Design XML Wednesday, October 20, 2004

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Lecture 10:Database Design

XML

Wednesday, October 20, 2004

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Outline

• Design of a Relational schema (3.6)

• XML

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Normal Forms

First Normal Form = all attributes are atomic

Second Normal Form (2NF) = old and obsolete

Third Normal Form (3NF) = this lecture

Boyce Codd Normal Form (BCNF) = this lecture

Others...

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Boyce-Codd Normal Form

A simple condition for removing anomalies from relations:

In English (though a bit vague):

Whenever a set of attributes of R is determining another attribute, should determine all the attributes of R.

A relation R is in BCNF if:

If A1, ..., An B is a non-trivial dependency

in R , then {A1, ..., An} is a key for R

A relation R is in BCNF if:

If A1, ..., An B is a non-trivial dependency

in R , then {A1, ..., An} is a key for R

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BCNF Decomposition Algorithm

A’s OthersB’s

R1

Is there a 2-attribute relation that isnot in BCNF ?

Repeat choose A1, …, Am B1, …, Bn that violates the BNCF condition split R into R1(A1, …, Am, B1, …, Bn) and R2(A1, …, Am, [others]) continue with both R1 and R2

Until no more violations

Repeat choose A1, …, Am B1, …, Bn that violates the BNCF condition split R into R1(A1, …, Am, B1, …, Bn) and R2(A1, …, Am, [others]) continue with both R1 and R2

Until no more violations

R2

In practice, we havea better algorithm (next):

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BCNF Decomposition Algorithm

BCNF_Decompose(R) find X s.t.: X ≠X+ ≠ [all attributes] if (not found) then “R is in BCNF” else let Y = X+ - X let Z = [all attributes] - X+ decompose into R1(X Y) and R2(X Z) BCNF_Decompose(R1) BCNF_Decompose(R2)

BCNF_Decompose(R) find X s.t.: X ≠X+ ≠ [all attributes] if (not found) then “R is in BCNF” else let Y = X+ - X let Z = [all attributes] - X+ decompose into R1(X Y) and R2(X Z) BCNF_Decompose(R1) BCNF_Decompose(R2)

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Example BCNF DecompositionPerson(name, SSN, age, hairColor, phoneNumber)

SSN name, ageage hairColor

Iteration 1: PersonSSN+ = SSN, name, age, hairColorDecompose into: P(SSN, name, age, hairColor) Phone(SSN, phoneNumber)

Iteration 2: Page+ = age, hairColorDecompose: People(SSN, name, age) Hair(age, hairColor) Phone(SSN, phoneNumber)

Iteration 1: PersonSSN+ = SSN, name, age, hairColorDecompose into: P(SSN, name, age, hairColor) Phone(SSN, phoneNumber)

Iteration 2: Page+ = age, hairColorDecompose: People(SSN, name, age) Hair(age, hairColor) Phone(SSN, phoneNumber)

Find X s.t.: X ≠X+ ≠ [all attributes]

What isthe key ?

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Other Example

• R(A,B,C,D) A B, B C

• Iteration 1: X = A: A+= ABC– split R into R1(A,B,C) R2(A,D)

• Iteration 2: X = B: B+=BC– Split R into R3(B,C), R4(A,B), R2(A,D)

• What happens if at iteration 1 we pick X = AB ?

What isthe key ?

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3NF: A Problem with BCNF

Unit CompanyCompany, Product Unit

Unit CompanyCompany, Product Unit

Unit+ = Unit, Company

We loose the FD: Company, Product Unit !!

Unit Company Product

Unit Company Unit Product

Unit CompanyUnit Company

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So What’s the Problem?

No problem so far. All local FD’s are satisfied.Let’s put all the data back into a single table again:

Unit Company

Galaga99 UW

Bingo UW

Unit Product

Galaga99 Databases

Bingo Databases

Unit Company Product

Galaga99 UW Databases

Bingo UW Databases

Unit CompanyUnit Company

Company, Product UnitCompany, Product UnitViolates the FD:

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The Problem

• We started with a table R and FD

• We decomposed R into BCNF tables R1, R2, …with their own FD1, FD2, …

• We can reconstruct R from R1, R2, …

• But we cannot reconstruct FD from FD1, FD2, …

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Solution: 3rd Normal Form (3NF)

A simple condition for removing anomalies from relations:

A relation R is in 3rd normal form if :

Whenever there is a nontrivial dependency A1, A2, ..., An Bfor R , then {A1, A2, ..., An } a super-key for R, or B is part of a key.

A relation R is in 3rd normal form if :

Whenever there is a nontrivial dependency A1, A2, ..., An Bfor R , then {A1, A2, ..., An } a super-key for R, or B is part of a key.

Tradeoff:BCNF = no anomalies, but may lose some FDs3NF = keeps all FDs, but may have some anomalies

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3NF Decomposition Algorithm

3NF_Decompose(R) let K = [all attributes that are part of some key] find X s.t.: X+ - X - K ≠ and X+ ≠ [all attributes] if (not found) then “R is in 3NF” else let Y = X+ - X - K let Z = [all attributes] - (X Y) decompose into R1(X Y) and R2(X Z) 3NF_Decompose(R1) 3NF_Decompose(R2)

3NF_Decompose(R) let K = [all attributes that are part of some key] find X s.t.: X+ - X - K ≠ and X+ ≠ [all attributes] if (not found) then “R is in 3NF” else let Y = X+ - X - K let Z = [all attributes] - (X Y) decompose into R1(X Y) and R2(X Z) 3NF_Decompose(R1) 3NF_Decompose(R2)

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Example of 3NF decompositionR(A,B,C,D,E):R(A,B,C,D,E):

AB CC DD BD E

AB CC DD BD E

Keys: (need to compute X+, for several Xs) AB, AC, AD

K = {A, B, C, D}

Pick X = CC+ = BCDEC BDE is a BCNF violationFor 3NF: remove B, D (part of K):C E is a 3NF violationDecompose: R1(C, E), R2(A,B,C,D)

R1 is in 3NFR2 is in 3NF (because its keys: AB, AC, AD)

15BCNF

3NF v.s. BCNF DecompositionA B C D E F G H K

A B C D E E F G H K

E F G G H KA B C C D E

A B A B A B A B A B A B A BA B

3NF

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XML Outline

• XML (4.6, 4.7)– This lecture: syntax, semistructured data– Next lectures: DTDs, XPath, XQuery

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Additional Readings on XML

• XQuery from the Experts, Katz, Ed. – The reference on Xquery

• http://www.w3.org/XML/1999/XML-in-10-points• www.zvon.org/xxl/XMLTutorial/General/book_en

.html• http://db.bell-labs.com/galax/• Main source: www.w3.org (but hard to read)

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XML

• eXtensible Markup Language

• XML 1.0 – a recommendation from W3C, 1998

• Roots: SGML (a very nasty language).

• After the roots: a format for sharing data

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XML Data

• Relational data does not have a syntax– I can’t “give” you my relational database– Need to import it from other other syntax, like CSV (comma-

separated-values)

• XML = rich syntax for data– But XML is not relational: semistructured

• Usage:– Map any data to XML– Store it in files, exchange on the Web, etc.– Even query it directly, using XPath, XQuery

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From HTML to XML

HTML describes the presentation

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HTML

<h1> Bibliography </h1>

<p> <i> Foundations of Databases </i>

Abiteboul, Hull, Vianu

<br> Addison Wesley, 1995

<p> <i> Data on the Web </i>

Abiteoul, Buneman, Suciu

<br> Morgan Kaufmann, 1999

<h1> Bibliography </h1>

<p> <i> Foundations of Databases </i>

Abiteboul, Hull, Vianu

<br> Addison Wesley, 1995

<p> <i> Data on the Web </i>

Abiteoul, Buneman, Suciu

<br> Morgan Kaufmann, 1999

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XML<bibliography>

<book> <title> Foundations… </title>

<author> Abiteboul </author>

<author> Hull </author>

<author> Vianu </author>

<publisher> Addison Wesley </publisher>

<year> 1995 </year>

</book>

…

</bibliography>

<bibliography>

<book> <title> Foundations… </title>

<author> Abiteboul </author>

<author> Hull </author>

<author> Vianu </author>

<publisher> Addison Wesley </publisher>

<year> 1995 </year>

</book>

…

</bibliography>

XML describes the content

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XML Terminology• tags: book, title, author, …• start tag: <book>, end tag: </book>• elements:

<book>…</book>,<author>…</author>• elements are nested• empty element: <red></red> abbrv. <red/>• an XML document: single root element

well formed XML document: if it has matching tags

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More XML: Attributes

<book price = “55” currency = “USD”>

<title> Foundations of Databases </title>

<author> Abiteboul </author>

…

<year> 1995 </year>

</book>

<book price = “55” currency = “USD”>

<title> Foundations of Databases </title>

<author> Abiteboul </author>

…

<year> 1995 </year>

</book>attributes are alternative ways to represent data

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More XML: Oids and References

<person id=“o555”> <name> Jane </name> </person>

<person id=“o456”> <name> Mary </name>

<children idref=“o123 o555”/>

</person>

<person id=“o123” mother=“o456”><name>John</name>

</person>

<person id=“o555”> <name> Jane </name> </person>

<person id=“o456”> <name> Mary </name>

<children idref=“o123 o555”/>

</person>

<person id=“o123” mother=“o456”><name>John</name>

</person>

oids and references in XML are just syntax

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More XML: CDATA Section

• Syntax: <![CDATA[ .....any text here...]]>

• Example:

<example> <![CDATA[ some text here </notAtag> <>]]></example>

<example> <![CDATA[ some text here </notAtag> <>]]></example>

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More XML: Entity References

• Syntax: &entityname;

• Example: <element> this is less than &lt; </element>

• Some entities: &lt; <

&gt; >

&amp; &

&apos; ‘

&quot; “

&#38; Unicode char

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More XML: Processing Instructions

• Syntax: <?target argument?>

• Example:

• What do they mean ?

•<product> <name> Alarm Clock </name> <?ringBell 20?> <price> 19.99 </price></product>

•<product> <name> Alarm Clock </name> <?ringBell 20?> <price> 19.99 </price></product>

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More XML: Comments

• Syntax <!-- .... Comment text... -->

• Yes, they are part of the data model !!!

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XML Namespaces

• http://www.w3.org/TR/REC-xml-names (1/99)

• name ::= [prefix:]localpart

<book xmlns:isbn=“www.isbn-org.org/def”>

<title> … </title>

<number> 15 </number>

<isbn:number> …. </isbn:number>

</book>

<book xmlns:isbn=“www.isbn-org.org/def”>

<title> … </title>

<number> 15 </number>

<isbn:number> …. </isbn:number>

</book>

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<tag xmlns:mystyle = “http://…”>

…

<mystyle:title> … </mystyle:title>

<mystyle:number> …

</tag>

<tag xmlns:mystyle = “http://…”>

…

<mystyle:title> … </mystyle:title>

<mystyle:number> …

</tag>

XML Namespaces

• syntactic: <number> , <isbn:number>

• semantic: provide URL for schema

Belong to this namespace

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From Relational Data to XML Data

<persons><row> <name>John</name> <phone> 3634</phone></row> <row> <name>Sue</name> <phone> 6343</phone> <row> <name>Dick</name> <phone>

6363</phone></row></persons>

<persons><row> <name>John</name> <phone> 3634</phone></row> <row> <name>Sue</name> <phone> 6343</phone> <row> <name>Dick</name> <phone>

6363</phone></row></persons>

row row row

name name namephone phone phone

“John” 3634 “Sue” “Dick”6343 6363Persons

XML: persons

Name Phone

John 3634

Sue 6343

Dick 6363

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XML Data

• XML is self-describing

• Schema elements become part of the data– Reational schema: persons(name,phone)– In XML <persons>, <name>, <phone> are part

of the data, and are repeated many times

• Consequence: XML is much more flexible

• XML = semistructured data

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Semi-structured Data Explained

• Missing attributes:

• Could represent ina table with nulls

<person> <name> John</name> <phone>1234</phone> </person>

<person> <name>Joe</name></person>

<person> <name> John</name> <phone>1234</phone> </person>

<person> <name>Joe</name></person> no phone !

name phone

John 1234

Joe -

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Semi-structured Data Explained

• Repeated attributes

• Impossible in tables:

<person> <name> Mary</name> <phone>2345</phone> <phone>3456</phone></person>

<person> <name> Mary</name> <phone>2345</phone> <phone>3456</phone></person>

two phones !

name phone

Mary 2345 3456 ???

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Semistructured Data Explained

• Attributes with different types in different objects

• Nested collections (no 1NF)• Heterogeneous collections:

– <db> contains both <book>s and <publisher>s

<person> <name> <first> John </first> <last> Smith </last> </name> <phone>1234</phone></person>

<person> <name> <first> John </first> <last> Smith </last> </name> <phone>1234</phone></person>

structured name !