S emistructured Data & XML

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1 S emistructured Data & XML Database Systems, A Practical Approach to Design, Implementation and Management (Connolly & Begg, Ch. 29) XML Bible (Harold, Ch. 1) S lide:1 14/04/04 1

2 Overview Semistructured Data Definition Justification Example XML Definition History Why XML? Basic Syntax XML elements Container Elements Attributes Comments Advantages of XML S lide:2 14/04/04 2

3 Definition of S emis tructured Data Data that may be irregular or incomplete and have a structure that may change rapidly or unpredictably Connolly (2002) a novel semi-structured table of data S lide:3 14/04/04 3

4 Jus tification Database schema not appropriate for data exchange Need for flexible data format for data exchange XML: a standard for data representation and exchange on the WEB S lide:4 14/04/04 Semistructured data have gained importance for the following reasons: it may be desirable to treat web sources like a database, but we cannot constrain these sources with a schema it may be desirable to have flexibility format fir data exchange between disparate databases the emergence of XML as the standard for data representation and exchange on the WEB and the similarity between XML and semistructured data. Connolly and Begg 4

5 Dream Home Databas e Structured data branchno street city postocode B Deer Rd London SW1 4EH B Agryll St Aberdeen AB2 3SU B Main St Glasgow G11 9QX staffno fname Sname Salary Position branchno SL21 John White Manager B005 SG37 Ann Beech Assistant B005 S lide:5 14/04/04 5

6 Dream Home Databas e Semitructured data Dream Home (&1) Branch (&2) Street (&5) 22 Deer Rd Manager &3 Staff (&3) Name (&6) fname (&9) John sname (&10) White ManagerOf &2 Staff (&4) Name (&7) Ann Beech Salary (&8) 1200 S lide:6 14/04/04 The example above is a sample representation of semistructured data following the object exchange model (OEM) which was developed to support data integration from different sources. The model is very similar to the hierarchical model as it uses the idea of nesting objects. Every object identified by a unique identifier. In the above example we can see data that is irregular and incomplete. For example, the name for staff &3 is shown in a different way to the name staff &4. Also, staff &4 has salary shown, whereas staff &3 does not. 6

7 Dream Home Databas e Dream Home &1 Branch Staff Staff &2 &3 &4 Street Name Name Salary &5 &6 &7 &8 22 Dear Road fname sname Ann Beech &9 &10 John White S lide:7 14/04/04 OEM structures data in a tree like hierarchy. XML follows the same approach. 7

8 Overview Semistructured Data Definition Justification Example XML Definition History Why XML? Basic Syntax XML elements Container Elements Attributes Comments Advantages of XML S lide:8 14/04/04 8

9 extens ible Markup Language (XML) Definition A meta-language (a language of describing other languages) that enables designers to create their customised tags to provide functionality not available with HTML. Connolly & Begg S lide:9 14/04/04 9

10 extens ible Markup Language (XML) XML provides the means for self description. XML is flexible so you can specify your markup language with its own tags and structures e.g. MathML, SMIL (i.e a metalanguage). Unlike HTML, XML itself intended for information management not display. S lide:10 14/04/04 ŒXML is a meta-markup language that defines a syntax used to define other mark up languages. A mark up language is a set of commands that tell a program how to display content. Content is words, pictures and other digitised data. ŒXML is a language in which you make up the tags as you go along. Of course, these tags must be organised according to certain rules but they are quite flexible in their meaning. For example, if you are working in chemistry you need tags to describe atoms. ŒThe XML basic syntax includes (i) the XML declaration (ii) elements (otherwise knows as tags) and attributes. The XML declaration provides information about the author of the document, the version of XML and whether or not there are external markup declarations referenced. Elements or tags are the most common form of markup. Finally, attributes are name-value pairs that contain descriptive information about an element (Connolly and Begg, 2002) ŒUnlike HTML, which defines a fixed number of tags that describe a fixed number of elements XML can be used to define unlimited number of tags. Also, unlike HTML which provides the facilities for formatting a document, including images in documents, linking a document to other documents and interacting with user input through forms, XML provides a means for self description where the markup is descriptive and the tags are more informative than just for formatting purposes. ŒXML describes document structure and meaning. It does not describe the formatting of the elements on the page. Formatting can be added with a style sheet. This is different to HTML, which has tags for describing formatting. For example tag <B> stands for bold. 10

11 History of XML mid 1980s: SGML (an extremely complex markup language to describe documents) 1989 HTML 1996 W3C design a simplified SGML that can overcome many of the limitations of HTML 1998 working drafts / recommendations for most aspects of XML S lide:11 14/04/04 XML emerged as a way to overcome the shortcomings of its two predecessors, SGML and HTML which were both very successful markup languages, but which were both flawed in certain ways. SGML, the international standard for marking up data, has been used since the 80s. SGML is an extremely powerful and extensible tool for semantic markup. However, SGML is pretty complex, especially for the everyday uses of the web. Not only that, but SGML is pretty expensive. Adding SGML capability to a word processor could double or triple the price. Finally, the commercial browsers made it pretty clear that they did not intend to ever support SGML. HTML on the other hand was free, simple and widely supported. HTML was originally designed at CERN around 1990 to provide a very simple version of SGML which could be used by "regular" people. As everyone knows, HTML became very popular. Unfortunately, HTML had serious defects that we discussed earlier. So in 1996, discussions began which focused on how to define a markup language with the power and extensibility of SGML but with the simplicity of HTML. The World Wide Web Consortium (W3C) decided to sponsor a group of SGML gurus to produce a simpler version of SGML. This team did to SGML what the Java team had done to C++. All of the non-essential, unused, cryptic parts of SGML were sliced away. What remained was a lean, mean marking up machine:xml. The specification of XML was only 26 pages as opposed to the 500+ pages of the SGML specification! Nevertheless, all the useful things which could be done by SGML, could also be done with XML. Over the next few years, XML evolved, drawing from the work of its sponsors and the work of developers solving similar problems such as Peter Murray-Rust who had been working on CML (Chemical Markup Language) and the consortium of group working on MathML. Finally, in 1998, the W3C approved Version 1.0 of the XML specification and a new language was born. 11

12 Why XML? (1) Enormous quantity of information outside of databases (e.g. memos, annual reports, poetry, recipes, home pages, books and magazines). So far resisted systematic storage. XML deals with this kind of information, providing structures that are firm enough to manage and exchange the information but flexible enough to allow authors to create documents that reflect their needs. S lide:12 14/04/04 It a language in which you make up the tags as you go along. Of course, these tags must be organised according to certain rules but they are quite flexible in their meaning. For example, if your working in chemistry you need tags to describe atoms. 12

13 S imple XML Example prolog root element <?xml version="1.0"?> <!DOCTYPE TEACHINGSCHEME SYSTEM "TeachingScheme.dtd"> <TEACHINGSCHEME> <TITLE>Databases</TITLE> <WEEK>1 Introduction</WEEK> <WEEK>2 Advanced databases</week> </TEACHINGSCHEME> <?...?> = an instruction to the XML processor S lide:13 14/04/04 Markup tags can have three kind of meanings Structure Semantics Style Structure divides documents into a tree of elements. Semantics relate the individual document to the real world outside the document. It is better to pick tags that more closely reflect the meaning of information they display. Style specifies how a document is displayed. The style meaning of a tag specifies how the content of the tag will be displayed. For example if an element will bold, underlined etc. In XML style meaning is applied through style sheets. 13

14 Basic S yntax declaration element tags attributes comments comments S lide:14 14/04/04 XML documents begin with an optional XML declaration, which in the example above indicates the version of XML used to author the document (1.0). In the example above, the declaration also includes the DTD (data type definition) which is discussed in the second XML lecture. Elements are the things in the document, such as paragraphs, tables, forms and images. With XML you can define your own elements as you want. Elements are defined using what we call tags. Attributes provide elements with properties such as width, height, or number of rows. Again, with XML you can define whatever attributes you want to define your elements. 14

15 XML Elements (1) logical structure information content <WEEK>1 Introduction</WEEK> markup markup S lide:15 14/04/04 15

16 XML Elements (2) Empty elements are denoted: <WEEK></WEEK> or <WEEK/> S lide:16 14/04/04 Empty tags Empty tags can be used as long as it is known that the element has no content. Rather than including a start and an end tag one can use an empty tag. 16

17 Container E lements <TEACHINGSCHEME> <TITLE>Databases</TITLE> <WEEK>1 Introduction</WEEK> <WEEK>2 Advanced databases</week> </TEACHINGSCHEME> Note the nesting of elements, this enables hierarchies to be constructed. S lide:17 14/04/04 Every good XML document must have a root element that contains every other element of the document. 17

18 Attributes These are name-value pairs relating to elements e.g. <WEEK weekno= 1 >Introduction</WEEK> Attributes placed in start tag. The above example could alternatively be represented with weekno as a sub element nested within WEEK. S lide:18 14/04/04 An attribute is a name-value pair that contains descriptive information about an element. The name and the value of the pair are both strings. An element may have a number of attributes but never two attributes of the same element are allowed to have the same name. There are no hard rules about when to use child elements and when to use attributes. Generally with experience users get a feel when attributes are easier to use than child elements and vice versa. An attribute is part of what we call metadata. To differentiate between data and metadata ask yourself whether someone reading the document would want to see a particular piece of information if the answer is yes that probably belongs in a child element. If the answer is no then the information belongs in an attribute. Attributes are good places to put id numbers. 18

19 Comments <TEACHINGSCHEME>  <TITLE>Databases</TITLE> <WEEK>1 Introduction</WEEK> <WEEK>2 Advanced databases</week> </TEACHINGSCHEME>  S lide:19 14/04/04 19

20 Advantages of XML Simplicity Open standard and platform/vendor independent Extensibility Reuse Separation of content and presentation Improved load balancing Support for the integration of data from multiple sources Ability to describe data from a wide variety of applications More advanced search engines New opportunities Connolly & Begg S lide:20 14/04/04 1. Simplicity XML is a relatively simple standard less than 50 pages long. Also information coded in XML is easy to read and understand. 2. Open standard and platform/vendor independent XML is both platform and vendor independent and a restricted form of SGML 3. Extensibility Unlike XML, HTML is extensible allowing users to define their own tags. 4. Reuse Extensibility also allows libraries of XML tags to be built once and reused by many applications. 5. Separation of content and Presentation XML separates the content of a document from how the document will be presented for example on the web. Facilities like stylesheets allow the same XML document to be published in different ways using a variety of formats and media. 6. Improved load balancing Data can be delivered to the browser on the desktop for local computation, offloading computation from the server and improving therefore load balancing. 7. Support for the integration of data from multiple sources XML enables data from different sources to be combined more easily. For example, database data can be easily translated to XML data and integrated with another database. 8. Ability to describe data from a wide variety of applications As XML is extensible it can be used to describe data contained in a variety of applications. 9. More advanced search engines With XML search engines will be able to parse the description bearing tags. 10. New opportunities 20

Chapter 2 XML, XML Schema, XSLT, and XPath

Summary Chapter 2 XML, XML Schema, XSLT, and XPath Ryan McAlister XML stands for Extensible Markup Language, meaning it uses tags to denote data much like HTML. Unlike HTML though it was designed to carry