Using Slidy

The slides for this class are HTML files which both validate as HTML 5 and pass the WAVE Accessibility Checker.
They are made to look like slides using a Javascript called Slidy.
The following keystrokes do useful things in Slidy:
- h - help (see all the commands)
- f - fullscreen (gets rid of the links at the bottom of the window
- space - advance a slide
- left/right arrows - forward or back a slide
- up/down arrows - scroll within a slide
- a - show all slides at once for printing
- u - up to the list of lectures

Semi-Structured Data

Recall a data model is a notation for describing data or information.
A typical data model needs to describe: the structure of the data, the operations on the data, and the constraints on the data.
In a first database course such as CS157A, we mainly focus on the relational data model.
Its base structure is a table of columns and rows called a relation.
It typically uses a language such as SQL for its operations and to specify the constraints on the data inserted into the model.
This semester we are going to begin by looking at a particular concrete instance of a semi-structured data model called XML.
A semi-structured data model is a model 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 are going to first spend some time describing how data is stored in XML and then go into how we can use XML with a database and how we can query data stored in XML (and later another semi-structured data format, JSON).

XML

XML traces its origins to the late 90s and attempts to simplify the specification of HTML (Hypertext Mark-up Language), the language used for web pages.
HTML was originally specified in a complicated language called SGML (Standard Generalized Markup Language) doctype that was better designed for specifying typesetting languages.
HTML only provides limited semantic information about a document so SGML was overkill. For example, you can tell if something is in <h1> tags that is probably important, but not much else.
Starting in 1997, a stripped-down version of SGML called XML (extensible markup language) was developed by the W3C (World-wide web consortium) to make it easier to create new tag-based mark-up languages where the tags can be used to carry whatever semantic information is desired.

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.

Semi-Structured Data, XML

Outline

Using Slidy

Semi-Structured Data

XML

An XML Document