Introduction

On Monday, we only had a short lecture as we mainly went over the syllabus.
We defined a data model as a notation for describing data or information.
Such as model needs to describe: the structure of the data, the operations on the data, and the constraints on the data.
We said that most intro database classes focus on the relational data model which stores data in relation (tables) and which might use relational algebra/SQL to define its operations/constraints.
We said this semester we were going to begin by considering semi-structured data models -- those where the data can typically be organized as some kind of tree or graph and the contents of the nodes in the tree or graph on not necessarily strictly typed.
We said XML provides an example of a semi-structure model and gave a little history on how XML was motivated by the desire to be able to easily define new tag-based languages for the web.
For the rest of today we are going to go into XML in detail. On Monday, we will start to consider how XML can be used with databases.

An XML Document

XML documents are text-based documents usually in some text encoding such as UTF-8 or ASCII.
To specify this encoding they might begin with a processing instruction (a sequence <? ... ?>) like:
```
<?xml version="1.0" encoding="UTF-8" ?>
```
Data in XML is stored in tags.
These are sequences of text beginning with a start tag, a text sequence that looks like:
```
<name_of_tag ... >
```
followed by some text contents which may or may not involve other tags, followed by a close tag that looks like:
```
</name_of_tag ... >
```

For example,

<Movies> 
   <Movie> 
   <title>Gone With the Wind</title>
   <Year>1939</Year> 
   <Length>231</Length>
   <Genre>drama</Genre>
   </Movie> 
   <Movie> 
   <title>Star Wars</title>
   <Year>1977</Year> 
   <Length>124</Length>
   <Genre>sciFi</Genre>
   </Movie>
   <Movie> 
   <title>Wayne's World</title>
   <Year>1992</Year>
   <Length>95</Length>
   <Genre>comedy</Genre>
   </Movie>
</Movies>

Start tags in XML are allowed to have attributes of the form attribute_name="attribute_value" which can further specify the meaning of the tag. For example,
```
<email use="work">chris.pollett@sjsu.edu</email>
```
HTML is a particular XML language with tags suitable for describing Web pages.
The power of XML comes from its use to define new tag based languages for the context needed. For example, the Movies document is an example of an instance of a different XML language suitable for describing movies.

The Syntax of XML

A new XML language can be specified in one of several ways (giving a DTD - Document Type Definition, using XML Schemas, using Relax-NG, etc.).
We will look at two ways, DTDs and then XML Schemas.
To create a new language with a DTD one specifies:
1. What tag elements exist in the language
2. What sub-elements or data a given element is allowed to contain
3. What attributes an element has and what their values can be.
4. What abbreviation entities are allowed in the language.
Tags are case-sensitive in XML, and every tag must have a close tag, although the abbreviation <element /> works as an implicit close tag.

XML Document Structure and Entities

Let's look at an XML document:

<patient type="out of state">
     <![CDATA[This is data that will not be parsed by the XML parser even if it has tags in it like this: <tag>]]>
<name><first>John</first><last>Smith</last></name>
<insurerID>&kaiser;</insurerID>
</patient>

Two files are usually associated with such a document:
1. one specifies what tags will be used with this document
2. another specifies how those tags should be styled.
One can abbreviate parts of a document using entities. For example, &kaiser; above might abbreviate some string of numbers.
Associated with any XML document is a document entity. This can be thought of as the root of the tree associated with the tags of the document. This entity thus can represent the whole document or the parts of it which do not change.
Document entities are often used to include part of spec for an XML language stored in one file into a whole spec file (for example in modular definitions of HTML).
Entities can also be used to reference binary data such as images.

Document Type Definitions

A DTD is used to specify:
1. what tags will be used in a document
2. what kind of data or subtags these tags can hold
3. what attributes the tags can have and what values for these attributes are legal
4. what entities can occur in the document
DTDs can be internal (occur in the XML document itself) or external (occur in some separate file).
A DTD consists of a sequence of declarations enclosed in the block of a DOCTYPE markup declaration. Items 1-4 above correspond to the declaration of type <!ELEMENT ...> (1 and 2), <!ATTLIST ...>, and <!ENTITY ...>
There is also a tag called <!NOTATION ...>which is sometimes used.

<!ELEMENT ..>

Basic syntax of the tag is:

<!ELEMENT element_name 
   (list of names of child elements) SYSTEM Location NDATA NotationName >

Some examples:

<!ELEMENT memo (from, to, date) > 
---- SYSTEM and NDATA don't have to used
, is used to represent a sequence where order matters. 
Use | if order  doesn't matter

<!ELEMENT dept_script SYSTEM "dept.php" NDATA "php" >
<!NOTATION php SYSTEM "/usr/bin/php" >

<!ELEMENT person(parent+, age, spouse?, sibling*) > 
---- + is one or more, ? is optional, and * is 0 or more

<!ELEMENT element_name (#PCDATA) > 
---- pcdata = parsable character data (text no tags).can Other 
possibilities are EMPTY  for no sub-tags or character data or ANY if 
you want to allow everything

<!ATTLIST ..>

The general way to specify a list of attributes is:

<!ATTLIST element_name 
attribute_name_1 attribute_type [default_value] ... attribute_name_n attribute_type [default_value]  >

For example,

<!ATTLIST airplane places CDATA "4">
<!ATTLIST airplane engine_type CDATA #REQUIRED> ---- must have the field
<!ATTLIST airplane price CDATA #IMPLIED> ---- no default value is given
<!ATTLIST airplane manufacturer CDATA #FIXED "cessna"> 
   --- all instances must have the same value

We can define multiple attributes in the same ATTLIST:

<!ATTLIST img src CDATA #REQUIRED
                 width  CDATA #REQUIRED
                 height CDATA #REQUIRED
                 alt    CDATA #IMPLIED
>

We can also list possible values rather than leave it as general as just CDATA:
```
<!ATTLIST horizontal_rule alignment (left|center|right) #IMPLIED >
```

<!ENTITY ..>

Entities come in two flavors:
- General entities that can be referenced anywhere in an XML document
- Parameter entities which can be referenced only in markup declarations
The general form of an entity declaration is:
```
<!ENTITY [%] entity_name "entity value" >
```
% is used when it is a parameter entity

For example,

<!ENTITY cp "Chris Pollett" >
<!ENTITY cool_pic SYSTEM "/usr/local/cool_pic.jpg">

Example DTD

<?xml version = "1.0" encoding ="utf-8" ?>
<!-- planes_for_sale.dtd  -->
<!ELEMENT planes_for_sale (ad+) >
<!ELEMENT ad (year, make, model, color, price?, seller) >
<!ELEMENT year (#PCDATA)>
<!ELEMENT make (#PCDATA)>
<!ELEMENT model (#PCDATA)>
<!ELEMENT color (#PCDATA)>
<!ELEMENT price (#PCDATA)>
<!ELEMENT seller (#PCDATA)>
<!ATTLIST seller license CDATA #REQUIRED >
<!ENTITY c "cessna" >

In-Class Exercise

Suppose you work for a web shopping cart company and have been tasked with developing an XML language suitable for an online shopping cart.
Documents in your language represent the contents of one shopping cart. Eventually we'd would like to store these in a database, but not for this exercise.
A shopping cart might have 0 or more items, each with an ID, a type, and a price and a vendor.
A single shopping cart belongs to some user.
One or more pre-built "deal" sequence of items exist.
Design a small XML language for this situation which makes use of <!ELEMENT ... > , <!ATTLIST ... > and <!ENTITY ... > declarations.
I have already created an account on the class discussion board for you and added you to the class discussion group. Your login is your surname followed by the first two letters of your personal name. For example, for me: pollettch . Your password has been initially set to changeme123 .
Post your solution to the Aug 26 In-Class Exercise Thread.

Internal versus External DTDs

There are two ways to associate a DTD with an XML document:

Internally --

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE planes_for_sale.dtd [ <!--DTD for planes_for_sale-->]>
<!-- The planes_for_sale document -->

Externally --

<!DOCTYPE planes_for_sale SYSTEM "planes_for_sale.dtd" >

Here is an example complete document that would validate against our planes_for_sale.dtd of a couple slides back

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE planes_for_sale SYSTEM "planes_for_sale.dtd" >
<planes_for_sale>
    <ad>
        <year>1960</year>
        <make>&c;</make>
        <model>150</model>
        <color>white</color>
        <seller license="12345678">Duran Duran</seller>
    </ad>
    <ad>
        <year>1945</year>
        <make>Douglas Aircraft Company</make>
        <model>DC-3</model>
        <color>red</color>
        <seller license="ABCDEFG">Gyro Gearloose</seller>
    </ad>
</planes_for_sale>

Namespaces

It is sometimes useful to mix and match markup from several XML languages into one file.
For instance, you might want to have mark-up for HTML as well as mark-up for SVG, a vector graphics language, and MathML, a language for mathematics.

One could do this with a declaration like:

<div xmlns="http://www.w3.org/1999/xhtml"
     xmlns:svg="http://www.w3.org/2000/svg"
	xmlns:mml="http://www.w3.org/1998/Math/MathML" >

Now to use an html tag within this div I don't need a prefix.
To use the svg tag rect, I might write <svg:rect ...>
This solves the problem where one might have two languages both with the same name for a tag. For intances, both with a <table> tag.

XML DTDs, Schemas, and XSLT

Outline

Introduction

An XML Document

The Syntax of XML

XML Document Structure and Entities

Document Type Definitions

<!ELEMENT ..>

<!ATTLIST ..>

<!ENTITY ..>

Example DTD

In-Class Exercise

Internal versus External DTDs

Namespaces