1. XML and Non-Relational Schemas
Zachary G. Ives
University of Pennsylvania
CIS 550 – Database & Information Systems
November 23, 2018
Some slide content courtesy of Susan Davidson & Raghu Ramakrishnan

2. Announcements
Homework 3 due Wednesday 10/21 at 4:30
Midterm: 10/26

3. Recall: Two Important Normal Forms
Boyce-Codd Normal Form (BCNF). For every relation scheme R and for every X  A that holds over R,
either A  X (it is trivial) ,or
or X is a superkey for R
Third Normal Form (3NF). For every relation scheme R and for every X  A that holds over R,
either A  X (it is trivial), or
X is a superkey for R, or
A is a member of some key for R
BCNF: the only functional dependencies are from the key. Partial dependencies aren’t allowed: if AB  CD and B  D then we fail on B  R. Ditto for our previous example, which was of the form A  C, BC  A.
3NF: the same but we allow for partial dependencies. This allows for the above example to be expressed as (A,B,C) because A  R and B & C both  R.
We still disallow partial keys, AB  CD, B  D because B isn’t a key for R. We also disallow transitive keys, AB  C, C  D because C isn’t a key for R.

4. BCNF Decomposition Algorithm (from Korth et al
BCNF Decomposition Algorithm (from Korth et al.; our book gives a recursive version)
result := {R}
compute F+
while there is a relation schema Ri in result that isn’t in BCNF {
let A  B be a nontrivial FD on Ri s.t. A  Ri is not in F+ and A and B are disjoint
result:= (result – Ri)  {(Ri - B), (A,B)} }
i.e., A doesn’t form a key
R = (sid, name, serno, cid, subj, exp-grade)
Compute F+:
sid  name
sid, serno  exp-grade
serno  cid, subj
cid  subj
sid, serno  R
(many trival deps, augmented deps)
Step 1: result := Ra(sid, serno, cid, subj, exp-grade) U R1(sid, name)
Step 2: result := Rb(sid, serno, cid, subj) U R2(sid, serno, exp-grade)
U R1(sid, name)
Step 3: result := Rc(sid, serno) U R3a(serno, cid, subj) U
R2(sid, serno, exp-grade) U R1(sid, name)
Step 4: result := Rc(sid, serno) U R4(cid, subj) U R3b(serno, cid) U
Can get different decomp with different ordering; can also get redundancy across relations (e.g., Rc)

5. An Example Given the schema: And FDs:
Stuff(sid, name, serno, classroom, cid, fid, prof)
And FDs:
sid  name serno  classroom, cid, fid
fid  prof
Find the Boyce-Codd Normal Form for this schema
What if instead:
sid  name classroom, cid  serno fid  prof serno  cid

6. 3NF Decomposition Algorithm
Let F be a minimal cover
i:=0
for each FD A  B in F {
if none of the schemas Rj, 1 j  i, contains AB {
increment i
Ri := (A, B) }
if no schema Rj, 1  j  i contains a candidate key for R {
Ri := any candidate key for R
return (R1, …, Ri)
Build dep.- preserving
decomp.
R = (sid, name, serno, cid, subj, exp-grade)
Minimal cover:
sid  name
serno  cid
cid  subj
sid, serno  exp-grade
R1 = (sid, name)
R2 = (serno, cid)
R3 = (cid, subj)
R4 = (sid, serno, exp-grade)
Ensure lossless decomp.

7. An Example Given the schema: And FDs:
Stuff(sid, name, serno, classroom, cid, fid, prof)
And FDs:
sid  name serno  classroom, cid, fid
fid  prof
Find the Third Normal Form for this schema
What if instead:
sid  name classroom, cid  serno fid  prof serno  cid

8. Normalization Is Good… Or Is It?
In some cases, we might not mind redundancy, if the data isn’t directly updated:
Reports (people like to see breakdowns by semester, department, course, etc.)
Warehouses (archived copies of data for doing complex analysis)
Data sharing (sometimes we may export data into object-oriented or hierarchical formats)

9. XML: A Semi-Structured Data Model

10. Why XML? XML is the confluence of several factors:
The Web needed a more declarative format for data
Documents needed a mechanism for extended tags
Database people needed a more flexible interchange format
“Lingua franca” of data
It’s parsable even if we don’t know what it means!
Original expectation:
The whole web would go to XML instead of HTML
Today’s reality:
Not so… But XML is used all over “under the covers”

11. Why DB People Like XML Can get data from all sorts of sources
Allows us to touch data we don’t own!
This was actually a huge change in the DB community
Interesting relationships with DB techniques
Useful to do relational-style operations
Leverages ideas from object-oriented, semistructured data
Blends schema and data into one format
Unlike relational model, where we need schema first
… But too little schema can be a drawback, too!

12. XML Anatomy Processing Instr. Open-tag Element Attribute Close-tag
<?xml version="1.0" encoding="ISO " ?>
<dblp>
<mastersthesis mdate=" " key="ms/Brown92">
  <author>Kurt P. Brown</author>
  <title>PRPL: A Database Workload Specification Language</title>
  <year>1992</year>
  <school>Univ. of Wisconsin-Madison</school>
  </mastersthesis>
<article mdate=" " key="tr/dec/SRC ">
  <editor>Paul R. McJones</editor>
  <title>The 1995 SQL Reunion</title>
  <journal>Digital System Research Center Report</journal>
  <volume>SRC </volume>
  <year>1997</year>
  <ee>db/labs/dec/SRC html</ee>
  <ee>
  </article>
Open-tag
Element
Attribute
Close-tag

13. Well-Formed XML A legal XML document – fully parsable by an XML parser
All open-tags have matching close-tags (unlike so many HTML documents!), or a special:
<tag/> shortcut for empty tags (equivalent to <tag></tag>
Attributes (which are unordered, in contrast to elements) only appear once in an element
There’s a single root element
XML is case-sensitive

14. XML as a Data Model XML “information set” includes 7 types of nodes:
Document (root)
Element
Attribute
Processing instruction
Text (content)
Namespace
Comment
XML data model includes this, plus typing info, plus order info and a few other things

15. XML Data Model Visualized (and simplified!)
root
attribute
p-i
element
Root
text
?xml
dblp
mastersthesis
article
mdate
mdate
key
key
2002…
author
title
year
school
editor
title
journal
volume
year ee ee
2002…
1992
1997
ms/Brown92
The…
tr/dec/…
PRPL…
Digital…
db/labs/dec
Kurt P….
Univ….
Paul R.
SRC…

16. What Does XML Do? Serves as a document format (super-HTML)
Allows custom tags (e.g., used by MS Word, openoffice)
Supplement it with stylesheets (XSL) to define formatting
Data exchange format (must agree on terminology)
Marshalling and unmarshalling data in SOAP and Web Services

17. XML as a Super-HTML (MS Word)
<h1 class="Section1"> <a name="_top“ />CIS 550: Database and Information Systems</h1>
<h2 class="Section1">Fall 2004</h2>
<p class="MsoNormal">
<place>311 Towne</place>, Tuesday/Thursday
<time Hour="13" Minute="30">1:30PM – 3:00PM</time>
</p>

18. XML Easily Encodes Relations
Student-course-grade
sid
serno
exp-grade 1
570103 B 23
550103 A
<student-course-grade>
<tuple><sid>1</sid><serno>570103</serno><exp-grade>B</exp-grade></tuple>
<tuple><sid>23</sid><serno>550103</serno><exp-grade>A</exp-grade></tuple>
</student-course-grade>

19. But XML is More Flexible… “Non-First-Normal-Form” (NF2)
<parents>
<parent name=“Jean” >
<son>John</son>
<daughter>Joan</daughter>
<daughter>Jill</daughter>
</parent>
<parent name=“Feng”>
<daughter>Felicity</daughter> …
Coincides with “semi-structured data”, invented by DB people at Penn and Stanford

20. XML and Code
Web Services (.NET, recent Java web service toolkits) are using XML to pass parameters and make function calls
Why?
Easy to be forwards-compatible
Easy to read over and validate (?)
Generally firewall-compatible
Drawbacks? XML is a verbose and inefficient encoding!
XML is used to represent:
SOAP: the “envelope” that data is marshalled into
XML Schema: gives some typing info about structures being passed
WSDL: the IDL (interface def language)
UDDI: provides an interface for querying about web services

21. Integrating XML: What If We Have Multiple Sources with the Same Tags?
Namespaces allow us to specify a context for different tags
Two parts:
Binding of namespace to URI
Qualified names
<root xmlns=“ xmlns:otherns=“…”>
<tag xmlns:myns=“
<thistag>is in the default namespace (aspace)</thistag>
<myns:thistag>is in myns</myns:thistag> <otherns:thistag>is a different tag in otherns</otherns:thistag>
</tag>
</root>

22. XML Isn’t Enough on Its Own
It’s too unconstrained for many cases!
How will we know when we’re getting garbage?
How will we query?
How will we understand what we got?
We also need:
Some idea of the structure
Our focus next
Presentation, in some cases – XSL(T)
We’ll talk about this soon
Some way of interpreting the tags…?
We’ll talk about this later in the semester

23. Structural Constraints: Document Type Definitions (DTDs)
The DTD is an EBNF grammar defining XML structure
XML document specifies an associated DTD, plus the root element
DTD specifies children of the root (and so on)
DTD defines special significance for attributes:
IDs – special attributes that are analogous to keys for elements
IDREFs – references to IDs
IDREFS – a nasty hack that represents a list of IDREFs

24. An Example DTD Example DTD: … Example use of DTD in XML file:
<!ELEMENT dblp((mastersthesis | article)*)>
<!ELEMENT mastersthesis(author,title,year,school,committeemember*)>
<!ATTLIST mastersthesis(mdate CDATA #REQUIRED key ID #REQUIRED
advisor CDATA #IMPLIED>
<!ELEMENT author(#PCDATA)> …
Example use of DTD in XML file:
<?xml version="1.0" encoding="ISO " ?>
<!DOCTYPE dblp SYSTEM “my.dtd">
<dblp>…

25. Representing Graphs and Links in XML
<?xml version="1.0" encoding="ISO " ?>
<!DOCTYPE graph SYSTEM “special.dtd">
<graph>
<author id=“author1”>
<name>John Smith</name>
</author>
<article>
<author ref=“author1” /> <title>Paper1</title>
</article>
<author ref=“author1” /> <title>Paper2</title> …

26. Graph Data Model Root graph ?xml !DOCTYPE article article author id
title
author
title
author
name
author1
Paper1
ref
Paper2
ref
John Smith
author1
author1

27. Graph Data Model Root graph ?xml !DOCTYPE article article author id
title
author
title
author
name
author1
Paper1
ref
Paper2
ref
John Smith

28. DTDs Aren’t Expressive Enough
DTDs capture grammatical structure, but have some drawbacks:
Not themselves in XML – inconvenient to build tools for them
Don’t capture database datatypes’ domains
IDs aren’t a good implementation of keys
Why not?
No way of defining OO-like inheritance

29. XML Schema Aims to address the shortcomings of DTDs XML syntax
Can define keys using XPaths
Type subclassing that’s more complex than in a programming language
Programming languages don’t consider order of member variables!
Subclassing “by extension” and “by restriction”
… And, of course, domains and built-in datatypes

30. Basics of XML Schema
Need to use the XML Schema namespace (generally named xsd)
simpleTypes are a way of restricting domains on scalars
Can define a simpleType based on integer, with values within a particular range
complexTypes are a way of defining element/attribute structures
Basically equivalent to !ELEMENT, but more powerful
Specify sequence, choice between child elements
Specify minOccurs and maxOccurs (default 1)
Must associate an element/attribute with a simpleType, or an element with a complexType

31. Simple Schema Example
<xsd:schema xmlns:xsd="
<xsd:element name=“mastersthesis" type=“ThesisType"/>
<xsd:complexType name=“ThesisType">
<xsd:attribute name=“mdate" type="xsd:date"/>
<xsd:attribute name=“key" type="xsd:string"/>
<xsd:attribute name=“advisor" type="xsd:string"/>
<xsd:sequence>
<xsd:element name=“author" type=“xsd:string"/>
<xsd:element name=“title" type=“xsd:string"/>
<xsd:element name=“year" type=“xsd:integer"/>
<xsd:element name=“school" type=“xsd:string”/>
<xsd:element name=“committeemember" type=“CommitteeType” minOccurs=“0"/>
</xsd:sequence>
</xsd:complexType>
</xsd:schema>

32. Designing an XML Schema/DTD
Not as formalized as relational data design
We can still use ER diagrams to break into entity, relationship sets
ER diagrams have extensions for “aggregation” – treating smaller diagrams as entities – and for composite attributes
Note that often we already have our data in relations and need to design the XML schema to export them!
Generally orient the XML tree around the “central” objects
Big decision: element vs. attribute
Element if it has its own properties, or if you *might* have more than one of them
Attribute if it is a single property – or perhaps not!

33. Recap: XML as a Data Model
XML is a non-first-normal-form (NF2) representation
Can represent documents, data
Standard data exchange format
Several competing schema formats – esp., DTD and XML Schema – provide typing information

34. Querying XML How do you query a directed graph? a tree?
The standard approach used by many XML, semistructured-data, and object query languages:
Define some sort of a template describing traversals from the root of the directed graph
In XML, the basis of this template is called an XPath

35. XPaths In its simplest form, an XPath is like a path in a file system:
/mypath/subpath/*/morepath
The XPath returns a node set representing the XML nodes (and their subtrees) at the end of the path
XPaths can have node tests at the end, returning only particular node types, e.g., text(), processing-instruction(), comment(), element(), attribute()
XPath is fundamentally an ordered language: it can query in order-aware fashion, and it returns nodes in order

36. Sample XML <?xml version="1.0" encoding="ISO-8859-1" ?>
<dblp>
<mastersthesis mdate=" " key="ms/Brown92">
  <author>Kurt P. Brown</author>
  <title>PRPL: A Database Workload Specification Language</title>
  <year>1992</year>
  <school>Univ. of Wisconsin-Madison</school>
  </mastersthesis>
<article mdate=" " key="tr/dec/SRC ">
  <editor>Paul R. McJones</editor>
  <title>The 1995 SQL Reunion</title>
  <journal>Digital System Research Center Report</journal>
  <volume>SRC </volume>
  <year>1997</year>
  <ee>db/labs/dec/SRC html</ee>
  <ee>
  </article>

37. XML Data Model Visualized
root
attribute
p-i
element
Root
text
?xml
dblp
mastersthesis
article
mdate
mdate
key
key
2002…
author
title
year
school
editor
title
journal
volume
year ee ee
2002…
1992
1997
ms/Brown92
The…
tr/dec/…
PRPL…
Digital…
db/labs/dec
Kurt P….
Univ….
Paul R.
SRC…

38. Some Example XPath Queries
/dblp/mastersthesis/title
/dblp/*/editor
//title
//title/text()

39. Context Nodes and Relative Paths
XPath has a notion of a context node: it’s analogous to a current directory
“.” represents this context node
“..” represents the parent node
We can express relative paths:
subpath/sub-subpath/../.. gets us back to the context node
By default, the document root is the context node

40. Predicates – Selection Operations
A predicate allows us to filter the node set based on selection-like conditions over sub-XPaths:
/dblp/article[title = “Paper1”]
which is equivalent to:
/dblp/article[./title/text() = “Paper1”]

41. Axes: More Complex Traversals
Thus far, we’ve seen XPath expressions that go down the tree (and up one step)
But we might want to go up, left, right, etc.
These are expressed with so-called axes:
self::path-step
child::path-step parent::path-step
descendant::path-step ancestor::path-step
descendant-or-self::path-step ancestor-or-self::path-step
preceding-sibling::path-step following-sibling::path-step
preceding::path-step following::path-step
The previous XPaths we saw were in “abbreviated form”

42. Querying Order
We saw in the previous slide that we could query for preceding or following siblings or nodes
We can also query a node for its position according to some index:
fn::first() , fn::last() return index of 0th & last element matching the last step:
fn::position() gives the relative count of the current node
child::article[fn::position() = fn::last()]

43. Users of XPath
XML Schema uses simple XPaths in defining keys and uniqueness constraints
XQuery
XSLT
XLink and XPointer, hyperlinks for XML