Information Management
Giovanni ̀ degli Studi di databases
• Characteristics
• Schema oriented
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• ACID properties
• Keep large amounts of persistent data
• Thanks to transactions, they limit the complexity of the management of concurrency
• Integration among multiple applications made possible through a shared database
• (Mostly) standard relational model
• Relational algebra for accessing data
Semi-structured data
(thanks to P. Samarati)
(some examples taken from Database Systems – Concepts, Languages and Architectures, P. Atzeni, S. Ceri, S. Paraboschi and R. Torlone, McGraw-Hill)
Semi-structured data
• Do not have a schema, or can respect it partly
• XML: eXtensible Markup Language
• Data properties expressed through markup of XML documents content
• A model for the data can be specified (DTD, XML Schema) • There are standardized query languages
• Data separated from metadata
• JSON: JavaScript Object Notation
• Derived from JavaScript
• Key-value pairs
• Data are organized in batches and nested objects
XML syntax
• XML files are composed of nested tags • Each XML document has a root tag
• All elements are enclosed in tags
• Elements
• Can be extended, can include children elements • Have a content (can be empty)
• Attributes
• Are included in elements, cannot have children
• Associate a value to elements without being part of their contents • Have limitations
XML – example (well-formed)
Namespaces and Schemas
• Namespaces avoid ambiguities for elements/attributes names • Collections of names identified by URI
•
• Schemas permit to define a model for documents
• Tags and their properties, cardinalities, domains, default values • A document can be validated w.r.t. a schema
• Schemas can be defined with DTD and XML Schema • DTD simpler, but XML Schema more flexible
• Permits to define elements and their cardinalities, and attributes •
• Can specify whether it contains children and their number
•
• Types and constraints (required, implied, fixed)
DTD – example
format (paperback|hardback) ‘paperback’
number CDATA #REQUIRED
XML Schema
• More flexible than DTD
• They are XML files themselves
• They permit to define
• Simple elements
• Complex elements
• Indicators (how often an element can occur) • Attributes
XML Schema – simple elements
• Simple elements
• Do not contain attributes nor other elements
• Can have fixed or default values
• Can be associated with restrictions (number of characters, sequence of characters, min/max value, list of values, …)
•
XML Schema – complex elements
• Complex elements
• Can contain attributes and other elements • Can have indicators
• any, all, choice, sequence: ordering among elements • {min,max}Occurs: min/max number of occurrences, grouping
•
XML Schema – attributes
• Attributes
• Can have default and fixed values
• Can be optional
• Can have restrictions (e.g., min/max value)
•
XML Schema – example
Querying XML documents
• Idea: XML as a tree structure to be visited
• Queries are based on XPath and FLWOR expression
• For, Let, Where, Order, Return
• Defines path expressions selecting the objects in a path
• Can return a set of nodes, a Boolean value, numbers, strings
• Paths are evaluated w.r.t. a context node
• Current node (.), parent node (..)
• Child of a node (/), descendant of a node (//), any node (*) • Attribute of current node predicate ([]), position ([n])
XPath – examples
doc(“books.xml”)/List/Book
doc(“books.xml”)/List/Book[Publisher= “Feltrinelli” AND @availability=“Y”] /Title
doc(“books.xml”)//Author doc(“books.xml”)/List/Book[1]/*
XQuery – FLWOR expressions
• FOR, LET
• They declare variables, difference in bindings
• It expresses conditions filtering objects produced by LET and FOR
• It orders objects produced by LET and FOR
• It generates the result of the query (node, forest, or value, and can include element constructors and references to variables)
XQuery – example (1)
FOR $lib IN doc(“books.xml”)//Book FOR $auth IN $lib/Author
RETURN $auth
LET $lib := doc(“books.xml”)//Book RETURN $lib
FOR $lib IN doc(“books.xml”)//Book WHERE $lib/Publisher=”Bompiani” AND
RETURN $lib
XQuery – example (2)
FOR $lib IN doc(“books.xml”)//Book WHERE $lib/Publisher=”Bompiani”
RETURN
FOR $lib IN doc(“books.xml”)//Book RETURN
XQuery – example (3)
let $author := doc(“catalog.xml”)//author
for $name in distinct-values($author/text())
order by $name
return
}
for $book in doc(“catalog.xml”)//libro
where $book/author/text()=$name
} return $book/title
XML Vs. DBMS
• RDBMS with extended capabilities
• Interfaces for translating XML to/from relational
• Interfaces for translating XML queries to relational queries • (Loose) mapping between schemas
• Native XML databases
• They lose RDBMS functionalities but natively manage XML structures (e.g.,
element inclusion, ordering among siblings)
• Appealing when data are poorly structured (e.g., schelamess)
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