XMLSemistructured DataThe Information-Integration ProblemExampleTwo Approaches to IntegrationWarehouse DiagramA MediatorGraphs of Semistructured DataExample: Data GraphSlide 10Well-Formed and Valid XMLWell-Formed XMLTagsExample: Well-Formed XMLXML and Semistructured DataSlide 16DTD StructureDTD ElementsExample: DTDElement DescriptionsExample: Element DescriptionUse of DTD’sExample (a)Example (b)AttributesExample: AttributesExample: Attribute UseID’s and IDREF’sCreating ID’sCreating IDREF’sExample: ID’s and IDREF’sThe DTDExample DocumentEmpty ElementsExample: Empty Element1XMLSemistructured DataExtensible Markup LanguageDocument Type Definitions2Semistructured DataAnother data model, based on trees.Motivation: flexible representation of data.Often, data comes from multiple sources with differences in notation, meaning, etc.Motivation: sharing of documents among systems and databases.3The Information-Integration ProblemRelated data exists in many places and could, in principle, work together.But different databases differ in:1. Model (relational, object-oriented?).2. Schema (normalized/unnormalized?).3. Terminology: are consultants employees? Retirees? Subcontractors?4. Conventions (meters versus feet?).4ExampleEvery bar has a database.One may use a relational DBMS; another keeps the menu in an MS-Word document.One stores the phones of distributors, another does not.One distinguishes ales from other beers, another doesn’t.One counts beer inventory by bottles, another by cases.5Two Approaches to Integration1. Warehousing : Make copies of the data sources at a central site and transform it to a common schema.Reconstruct data daily/weekly, but do not try to keep it more up-to-date than that.2. Mediation : Create a view of all sources, as if they were integrated.Answer a view query by translating it to terminology of the sources and querying them.6Warehouse DiagramWarehouseWrapper WrapperSource 1 Source 27A MediatorMediatorWrapper WrapperSource 1 Source 2User queryQueryQueryQueryQueryResultResultResultResultResult8Graphs of Semistructured DataNodes = objects.Labels on arcs (attributes, relationships).Atomic values at leaf nodes (nodes with no arcs out).Flexibility: no restriction on:Labels out of a node.9Example: Data GraphBudA.B.Gold1995MapleJoe’sMillerbeer beerbarmanfmanfservedAtnamenamenameaddrprizeyear awardrootThe bar objectfor Joe’s BarThe beer objectfor BudNotice anew kindof data.10XMLXML = Extensible Markup Language.While HTML uses tags for formatting (e.g., “italic”), XML uses tags for semantics (e.g., “this is an address”).Key idea: create tag sets for a domain (e.g., genomics), and translate all data into properly tagged XML documents.11Well-Formed and Valid XMLWell-Formed XML allows you to invent your own tags.Similar to labels in semistructured data.Valid XML involves a DTD (Document Type Definition), a grammar for tags.12Well-Formed XMLStart the document with a declaration, surrounded by <?xml … ?> .Normal declaration is:<?xml version = “1.0” standalone = “yes” ?>“Standalone” = “no DTD provided.”Balance of document is a root tag surrounding nested tags.13TagsTags, as in HTML, are normally matched pairs, as <FOO> … </FOO> .Tags may be nested arbitrarily.XML tags are case sensitive.14Example: Well-Formed XML<?xml version = “1.0” standalone = “yes” ?><BARS><BAR><NAME>Joe’s Bar</NAME><BEER><NAME>Bud</NAME><PRICE>2.50</PRICE></BEER><BEER><NAME>Miller</NAME><PRICE>3.00</PRICE></BEER></BAR><BAR> … </BARS>A NAMEsubobjectA BEERsubobject15XML and Semistructured DataWell-Formed XML with nested tags is exactly the same idea as trees of semistructured data.We shall see that XML also enables nontree structures, as does the semistructured data model.16ExampleThe <BARS> XML document is:Joe’s BarBud 2.50 Miller 3.00PRICEBARBARBARSNAME. . .BARPRICENAMEBEERBEERNAME17DTD Structure<!DOCTYPE <root tag> [<!ELEMENT <name>(<components>)>. . . more elements . . .]>18DTD ElementsThe description of an element consists of its name (tag), and a parenthesized description of any nested tags.Includes order of subtags and their multiplicity.Leaves (text elements) have #PCDATA (Parsed Character DATA ) in place of nested tags.19Example: DTD<!DOCTYPE BARS [<!ELEMENT BARS (BAR*)><!ELEMENT BAR (NAME, BEER+)><!ELEMENT NAME (#PCDATA)><!ELEMENT BEER (NAME, PRICE)><!ELEMENT PRICE (#PCDATA)>]>A BARS object haszero or more BAR’snested within.A BAR has oneNAME and oneor more BEERsubobjects.A BEER has aNAME and aPRICE.NAME and PRICEare text.20Element DescriptionsSubtags must appear in order shown.A tag may be followed by a symbol to indicate its multiplicity.* = zero or more.+ = one or more.? = zero or one.Symbol | can connect alternative sequences of tags.21Example: Element DescriptionA name is an optional title (e.g., “Prof.”), a first name, and a last name, in that order, or it is an IP address:<!ELEMENT NAME ((TITLE?, FIRST, LAST) | IPADDR)>22Use of DTD’s1. Set standalone = “no”.2. Either:a) Include the DTD as a preamble of the XML document, orb) Follow DOCTYPE and the <root tag> by SYSTEM and a path to the file where the DTD can be found.23Example (a)<?xml version = “1.0” standalone = “no” ?><!DOCTYPE BARS [<!ELEMENT BARS (BAR*)><!ELEMENT BAR (NAME, BEER+)><!ELEMENT NAME (#PCDATA)><!ELEMENT BEER (NAME, PRICE)><!ELEMENT PRICE (#PCDATA)>]><BARS><BAR><NAME>Joe’s Bar</NAME><BEER><NAME>Bud</NAME> <PRICE>2.50</PRICE></BEER><BEER><NAME>Miller</NAME> <PRICE>3.00</PRICE></BEER></BAR> <BAR> … </BARS>The DTDThe document24Example (b)Assume the BARS DTD is in file bar.dtd.<?xml version = “1.0” standalone = “no” ?><!DOCTYPE BARS SYSTEM “bar.dtd”><BARS><BAR><NAME>Joe’s Bar</NAME><BEER><NAME>Bud</NAME><PRICE>2.50</PRICE></BEER><BEER><NAME>Miller</NAME><PRICE>3.00</PRICE></BEER></BAR><BAR> … </BARS>Get the DTDfrom the filebar.dtd25AttributesOpening tags in XML can have attributes.In a DTD,<!ATTLIST E . . . > declares an attribute for element E, along with its datatype.26Example: AttributesBars can have an attribute kind, a character string describing the bar.<!ELEMENT BAR (NAME BEER*)><!ATTLIST BAR kind CDATA #IMPLIED>Character stringtype; no tagsAttribute is optionalopposite: #REQUIRED27Example: Attribute UseIn a document that allows BAR tags, we might see:<BAR kind =