Database System Concepts ©Silberschatz, Korth and Sudarshan See for conditions on re-usewww.db-book.com Chapter 10: XML

©Silberschatz, Korth and Sudarshan10.2Database System Concepts - 5 th Edition, Aug 22, XML Structure of XML Data XML Document Schema Querying and Transformation Application Program Interfaces to XML Storage of XML Data XML Applications

©Silberschatz, Korth and Sudarshan10.3Database System Concepts - 5 th Edition, Aug 22, Introduction XML: Extensible Markup Language Defined by the WWW Consortium (W3C) Derived from SGML (Standard Generalized Markup Language), but simpler to use than SGML Documents have tags giving extra information about sections of the document E.g. XML Introduction … Extensible, unlike HTML Users can add new tags, and separately specify how the tag should be handled for display

©Silberschatz, Korth and Sudarshan10.4Database System Concepts - 5 th Edition, Aug 22, XML Introduction (Cont.) The ability to specify new tags, and to create nested tag structures make XML a great way to exchange data, not just documents. Much of the use of XML has been in data exchange applications, not as a replacement for HTML Tags make data (relatively) self-documenting E.g. A-101 Downtown 500 A-101 Johnson

©Silberschatz, Korth and Sudarshan10.5Database System Concepts - 5 th Edition, Aug 22, XML: Motivation Data interchange is critical in todays networked world Examples: Banking: funds transfer Order processing (especially inter-company orders) Scientific data –Chemistry: ChemML, … –Genetics: BSML (Bio-Sequence Markup Language), … Paper flow of information between organizations is being replaced by electronic flow of information Each application area has its own set of standards for representing information XML has become the basis for all new generation data interchange formats

©Silberschatz, Korth and Sudarshan10.6Database System Concepts - 5 th Edition, Aug 22, XML Motivation (Cont.) Earlier generation formats were based on plain text with line headers indicating the meaning of fields Similar in concept to headers Does not allow for nested structures, no standard type language Tied too closely to low level document structure (lines, spaces, etc) Each XML based standard defines what are valid elements, using XML type specification languages to specify the syntax DTD (Document Type Descriptors) XML Schema Plus textual descriptions of the semantics XML allows new tags to be defined as required However, this may be constrained by DTDs A wide variety of tools is available for parsing, browsing and querying XML documents/data

©Silberschatz, Korth and Sudarshan10.7Database System Concepts - 5 th Edition, Aug 22, Comparison with Relational Data Inefficient: tags, which in effect represent schema information, are repeated Better than relational tuples as a data-exchange format Unlike relational tuples, XML data is self-documenting due to presence of tags Non-rigid format: tags can be added Allows nested structures Wide acceptance, not only in database systems, but also in browsers, tools, and applications

©Silberschatz, Korth and Sudarshan10.8Database System Concepts - 5 th Edition, Aug 22, Structure of XML Data Tag: label for a section of data Element: section of data beginning with and ending with matching Elements must be properly nested Proper nesting … …. Improper nesting … …. Formally: every start tag must have a unique matching end tag, that is in the context of the same parent element. Every document must have a single top-level element

©Silberschatz, Korth and Sudarshan10.9Database System Concepts - 5 th Edition, Aug 22, Example of Nested Elements Hayes Main Harrison A-102 Perryridge 400 …..

©Silberschatz, Korth and Sudarshan10.10Database System Concepts - 5 th Edition, Aug 22, Motivation for Nesting Nesting of data is useful in data transfer Example: elements representing customer_id, customer_name, and address nested within an order element Nesting is not supported, or discouraged, in relational databases With multiple orders, customer name and address are stored redundantly normalization replaces nested structures in each order by foreign key into table storing customer name and address information Nesting is supported in object-relational databases But nesting is appropriate when transferring data External application does not have direct access to data referenced by a foreign key

©Silberschatz, Korth and Sudarshan10.11Database System Concepts - 5 th Edition, Aug 22, Structure of XML Data (Cont.) Mixture of text with sub-elements is legal in XML. Example: This account is seldom used any more. A-102 Perryridge 400 Useful for document markup, but discouraged for data representation

©Silberschatz, Korth and Sudarshan10.12Database System Concepts - 5 th Edition, Aug 22, Attributes Elements can have attributes A-102 Perryridge 400 Attributes are specified by name=value pairs inside the starting tag of an element An element may have several attributes, but each attribute name can only occur once

©Silberschatz, Korth and Sudarshan10.13Database System Concepts - 5 th Edition, Aug 22, Attributes vs. Subelements Distinction between subelement and attribute In the context of documents, attributes are part of markup, while subelement contents are part of the basic document contents In the context of data representation, the difference is unclear and may be confusing Same information can be represented in two ways – …. – A-101 … Suggestion: use attributes for identifiers of elements, and use subelements for contents

©Silberschatz, Korth and Sudarshan10.14Database System Concepts - 5 th Edition, Aug 22, Namespaces XML data has to be exchanged between organizations Same tag name may have different meaning in different organizations, causing confusion on exchanged documents Specifying a unique string as an element name avoids confusion Better solution: use unique-name:element-name Avoid using long unique names all over document by using XML Namespaces … Downtown Brooklyn …

©Silberschatz, Korth and Sudarshan10.15Database System Concepts - 5 th Edition, Aug 22, More on XML Syntax Elements without subelements or text content can be abbreviated by ending the start tag with a /> and deleting the end tag To store string data that may contain tags, without the tags being interpreted as subelements, use CDATA as below … ]]> Here, and are treated as just strings CDATA stands for character data

©Silberschatz, Korth and Sudarshan10.16Database System Concepts - 5 th Edition, Aug 22, XML Document Schema Database schemas constrain what information can be stored, and the data types of stored values XML documents are not required to have an associated schema However, schemas are very important for XML data exchange Otherwise, a site cannot automatically interpret data received from another site Two mechanisms for specifying XML schema Document Type Definition (DTD) Widely used XML Schema Newer, increasing use

©Silberschatz, Korth and Sudarshan10.17Database System Concepts - 5 th Edition, Aug 22, Document Type Definition (DTD) The type of an XML document can be specified using a DTD DTD constraints structure of XML data What elements can occur What attributes can/must an element have What subelements can/must occur inside each element, and how many times. DTD does not constrain data types All values represented as strings in XML DTD syntax

©Silberschatz, Korth and Sudarshan10.18Database System Concepts - 5 th Edition, Aug 22, Element Specification in DTD Subelements can be specified as names of elements, or #PCDATA (parsed character data), i.e., character strings EMPTY (no subelements) or ANY (anything can be a subelement) Example Subelement specification may have regular expressions Notation: – | - alternatives – or more occurrences – * - 0 or more occurrences

©Silberschatz, Korth and Sudarshan10.19Database System Concepts - 5 th Edition, Aug 22, Bank DTD <!DOCTYPE bank [ ]>

©Silberschatz, Korth and Sudarshan10.20Database System Concepts - 5 th Edition, Aug 22, Attribute Specification in DTD Attribute specification : for each attribute Name Type of attribute CDATA ID (identifier) or IDREF (ID reference) or IDREFS (multiple IDREFs) – more on this later Whether mandatory (#REQUIRED) has a default value (value), or neither (#IMPLIED) Examples <!ATTLIST customer customer_id ID # REQUIRED accounts IDREFS # REQUIRED >

©Silberschatz, Korth and Sudarshan10.21Database System Concepts - 5 th Edition, Aug 22, IDs and IDREFs An element can have at most one attribute of type ID The ID attribute value of each element in an XML document must be distinct Thus the ID attribute value is an object identifier An attribute of type IDREF must contain the ID value of an element in the same document An attribute of type IDREFS contains a set of (0 or more) ID values. Each ID value must contain the ID value of an element in the same document

©Silberschatz, Korth and Sudarshan10.22Database System Concepts - 5 th Edition, Aug 22, Bank DTD with Attributes Bank DTD with ID and IDREF attribute types. <!DOCTYPE bank-2[ <!ATTLIST account account_number ID # REQUIRED owners IDREFS # REQUIRED> <!ELEMENT customer(customer_name, customer_street, customer_city)> <!ATTLIST customer customer_id ID # REQUIRED accounts IDREFS # REQUIRED> … declarations for branch, balance, customer_name, customer_street and customer_city ]>

©Silberschatz, Korth and Sudarshan10.23Database System Concepts - 5 th Edition, Aug 22, XML data with ID and IDREF attributes Downtown 500 ….. Joe Monroe Madison Mary Erin Newark

©Silberschatz, Korth and Sudarshan10.24Database System Concepts - 5 th Edition, Aug 22, Limitations of DTDs No typing of text elements and attributes All values are strings, no integers, reals, etc. Difficult to specify unordered sets of subelements Order is usually irrelevant in databases (unlike in the document- layout environment from which XML evolved) (A | B)* allows specification of an unordered set, but Cannot ensure that each of A and B occurs only once IDs and IDREFs are untyped The owners attribute of an account may contain a reference to another account, which is meaningless owners attribute should ideally be constrained to refer to customer elements

©Silberschatz, Korth and Sudarshan10.25Database System Concepts - 5 th Edition, Aug 22, XML Schema XML Schema is a more sophisticated schema language which addresses the drawbacks of DTDs. Supports Typing of values E.g. integer, string, etc Also, constraints on min/max values User-defined, comlex types Many more features, including uniqueness and foreign key constraints, inheritance XML Schema is itself specified in XML syntax, unlike DTDs More-standard representation, but verbose XML Scheme is integrated with namespaces BUT: XML Schema is significantly more complicated than DTDs.

©Silberschatz, Korth and Sudarshan10.26Database System Concepts - 5 th Edition, Aug 22, XML Schema Version of Bank DTD ….. definitions of customer and depositor ….

©Silberschatz, Korth and Sudarshan10.27Database System Concepts - 5 th Edition, Aug 22, XML Schema Version of Bank DTD Choice of xs: was ours -- any other namespace prefix could be chosen Element bank has type BankType, which is defined separately xs:complexType is used later to create the named complex type BankType Element account has its type defined in-line

©Silberschatz, Korth and Sudarshan10.28Database System Concepts - 5 th Edition, Aug 22, More features of XML Schema Attributes specified by xs:attribute tag: adding the attribute use = required means value must be specified Key constraint: account numbers form a key for account elements under the root bank element: Foreign key constraint from depositor to account:

©Silberschatz, Korth and Sudarshan10.29Database System Concepts - 5 th Edition, Aug 22, Querying and Transforming XML Data Translation of information from one XML schema to another Querying on XML data Above two are closely related, and handled by the same tools Standard XML querying/translation languages XPath Simple language consisting of path expressions XQuery An XML query language with a rich set of features XSLT Simple language designed for translation from XML to XML and XML to HTML

©Silberschatz, Korth and Sudarshan10.30Database System Concepts - 5 th Edition, Aug 22, Tree Model of XML Data Query and transformation languages are based on a tree model of XML data An XML document is modeled as a tree, with nodes corresponding to elements and attributes Element nodes have child nodes, which can be attributes or subelements Text in an element is modeled as a text node child of the element Children of a node are ordered according to their order in the XML document Element and attribute nodes (except for the root node) have a single parent, which is an element node The root node has a single child, which is the root element of the document

©Silberschatz, Korth and Sudarshan10.31Database System Concepts - 5 th Edition, Aug 22, XPath XPath is used to address (select) parts of documents using path expressions A path expression is a sequence of steps separated by / Think of file names in a directory hierarchy Result of path expression: set of values that along with their containing elements/attributes match the specified path E.g. /bank-2/customer/customer_name evaluated on the bank-2 data we saw earlier returnsbank-2 data Joe Mary E.g. /bank-2/customer/customer_name/text( ) returns the same names, but without the enclosing tags

©Silberschatz, Korth and Sudarshan10.32Database System Concepts - 5 th Edition, Aug 22, XPath (Cont.) The initial / denotes root of the document (above the top-level tag) Path expressions are evaluated left to right Each step operates on the set of instances produced by the previous step Selection predicates may follow any step in a path, in [ ] E.g. /bank-2/account[balance > 400] returns account elements with a balance value greater than 400 /bank-2/account[balance] returns account elements containing a balance subelement Attributes are accessed E.g. /bank-2/account[balance > returns the account numbers of accounts with balance > 400 IDREF attributes are not dereferenced automatically (more on this later)

©Silberschatz, Korth and Sudarshan10.33Database System Concepts - 5 th Edition, Aug 22, Functions in XPath XPath provides several functions The function count() at the end of a path counts the number of elements in the set generated by the path E.g. /bank-2/account[count(./customer) > 2] –Returns accounts with > 2 customers Also function for testing position (1, 2,..) of node w.r.t. siblings Boolean connectives and and or and function not() can be used in predicates IDREFs can be referenced using function id() id() can also be applied to sets of references such as IDREFS and even to strings containing multiple references separated by blanks E.g. returns all customers referred to from the owners attribute of account elements.

©Silberschatz, Korth and Sudarshan10.34Database System Concepts - 5 th Edition, Aug 22, More XPath Features Operator | used to implement union E.g. | Gives customers with either accounts or loans However, | cannot be nested inside other operators. // can be used to skip multiple levels of nodes E.g. /bank-2//customer_name finds any customer_name element anywhere under the /bank-2 element, regardless of the element in which it is contained. A step in the path can go to parents, siblings, ancestors and descendants of the nodes generated by the previous step, not just to the children //, described above, is a short from for specifying all descendants.. specifies the parent. doc(name) returns the root of a named document

©Silberschatz, Korth and Sudarshan10.35Database System Concepts - 5 th Edition, Aug 22, XQuery XQuery is a general purpose query language for XML data Currently being standardized by the World Wide Web Consortium (W3C) The textbook description is based on a January 2005 draft of the standard. The final version may differ, but major features likely to stay unchanged. XQuery is derived from the Quilt query language, which itself borrows from SQL, XQL and XML-QL XQuery uses a for … let … where … order by …result … syntax for SQL from where SQL where order by SQL order by result SQL select let allows temporary variables, and has no equivalent in SQL

©Silberschatz, Korth and Sudarshan10.36Database System Concepts - 5 th Edition, Aug 22, FLWOR Syntax in XQuery For clause uses XPath expressions, and variable in for clause ranges over values in the set returned by XPath Simple FLWOR expression in XQuery find all accounts with balance > 400, with each result enclosed in an.. tag for $x in /bank-2/account let $acctno := where $x/balance > 400 return { $acctno } Items in the return clause are XML text unless enclosed in { }, in which case they are evaluated Let clause not really needed in this query, and selection can be done In XPath. Query can be written as: for $x in /bank-2/account[balance>400] return { }

©Silberschatz, Korth and Sudarshan10.37Database System Concepts - 5 th Edition, Aug 22, Joins Joins are specified in a manner very similar to SQL for $a in /bank/account, $c in /bank/customer, $d in /bank/depositor where $a/account_number = $d/account_number and $c/customer_name = $d/customer_name return { $c $a } The same query can be expressed with the selections specified as XPath selections: for $a in /bank/account $c in /bank/customer $d in /bank/depositor[ account_number = $a/account_number and customer_name = $c/customer_name ] return { $c $a }

©Silberschatz, Korth and Sudarshan10.38Database System Concepts - 5 th Edition, Aug 22, Nested Queries The following query converts data from the flat structure for bank information into the nested structure used in bank-1 { for $c in /bank/customer return { $c/* } { for $d in /bank/depositor[customer_name = $c/customer_name], $a in /bank/account[account_number=$d/account_number] return $a } } $c/* denotes all the children of the node to which $c is bound, without the enclosing top-level tag $c/text() gives text content of an element without any subelements / tags

©Silberschatz, Korth and Sudarshan10.39Database System Concepts - 5 th Edition, Aug 22, Sorting in XQuery The order by clause can be used at the end of any expression. E.g. to return customers sorted by name for $c in /bank/customer order by $c/customer_name return { $c/* } Use order by $c/customer_name to sort in descending order Can sort at multiple levels of nesting (sort by customer_name, and by account_number within each customer) { for $c in /bank/customer order by $c/customer_name return { $c/* } { for $d in /bank/depositor[customer_name=$c/customer_name], $a in /bank/account[account_number=$d/account_number] order by $a/account_number return $a/* } }

©Silberschatz, Korth and Sudarshan10.40Database System Concepts - 5 th Edition, Aug 22, Functions and Other XQuery Features User defined functions with the type system of XMLSchema function balances(xs:string $c) returns list(xs:decimal*) { for $d in /bank/depositor[customer_name = $c], $a in /bank/account[account_number = $d/account_number] return $a/balance } Types are optional for function parameters and return values The * (as in decimal*) indicates a sequence of values of that type Universal and existential quantification in where clause predicates some $e in path satisfies P every $e in path satisfies P XQuery also supports If-then-else clauses

©Silberschatz, Korth and Sudarshan10.41Database System Concepts - 5 th Edition, Aug 22, XSLT A stylesheet stores formatting options for a document, usually separately from document E.g. an HTML style sheet may specify font colors and sizes for headings, etc. The XML Stylesheet Language (XSL) was originally designed for generating HTML from XML XSLT is a general-purpose transformation language Can translate XML to XML, and XML to HTML XSLT transformations are expressed using rules called templates Templates combine selection using XPath with construction of results

©Silberschatz, Korth and Sudarshan10.42Database System Concepts - 5 th Edition, Aug 22, XSLT Templates Example of XSLT template with match and select part The match attribute of xsl:template specifies a pattern in XPath Elements in the XML document matching the pattern are processed by the actions within the xsl:template element xsl:value-of selects (outputs) specified values (here, customer_name) For elements that do not match any template Attributes and text contents are output as is Templates are recursively applied on subelements The template matches all elements that do not match any other template Used to ensure that their contents do not get output. If an element matches several templates, only one is used based on a complex priority scheme/user-defined priorities

©Silberschatz, Korth and Sudarshan10.43Database System Concepts - 5 th Edition, Aug 22, Creating XML Output Any text or tag in the XSL stylesheet that is not in the xsl namespace is output as is E.g. to wrap results in new XML elements. Example output: Joe Mary

©Silberschatz, Korth and Sudarshan10.44Database System Concepts - 5 th Edition, Aug 22, Creating XML Output (Cont.) Note: Cannot directly insert a xsl:value-of tag inside another tag E.g. cannot create an attribute for in the previous example by directly using xsl:value-of XSLT provides a construct xsl:attribute to handle this situation xsl:attribute adds attribute to the preceding element E.g. results in output of the form …. xsl:element is used to create output elements with computed names

©Silberschatz, Korth and Sudarshan10.45Database System Concepts - 5 th Edition, Aug 22, Structural Recursion Template action can apply templates recursively to the contents of a matched element Example output: John Mary

©Silberschatz, Korth and Sudarshan10.46Database System Concepts - 5 th Edition, Aug 22, Joins in XSLT XSLT keys allow elements to be looked up (indexed) by values of subelements or attributes Keys must be declared (with a name) and, the key() function can then be used for lookup. E.g. <xsl:value-of select=key(acctno, A-101) Keys permit (some) joins to be expressed in XSLT

©Silberschatz, Korth and Sudarshan10.47Database System Concepts - 5 th Edition, Aug 22, Sorting in XSLT Using an xsl:sort directive inside a template causes all elements matching the template to be sorted Sorting is done before applying other templates

©Silberschatz, Korth and Sudarshan10.48Database System Concepts - 5 th Edition, Aug 22, Application Program Interface There are two standard application program interfaces to XML data: SAX (Simple API for XML) Based on parser model, user provides event handlers for parsing events –E.g. start of element, end of element –Not suitable for database applications DOM (Document Object Model) XML data is parsed into a tree representation Variety of functions provided for traversing the DOM tree E.g.: Java DOM API provides Node class with methods getParentNode( ), getFirstChild( ), getNextSibling( ) getAttribute( ), getData( ) (for text node) getElementsByTagName( ), … Also provides functions for updating DOM tree

©Silberschatz, Korth and Sudarshan10.49Database System Concepts - 5 th Edition, Aug 22, Storage of XML Data XML data can be stored in Non-relational data stores Flat files –Natural for storing XML –But has all problems discussed in Chapter 1 (no concurrency, no recovery, …) XML database –Database built specifically for storing XML data, supporting DOM model and declarative querying –Currently no commercial-grade systems Relational databases Data must be translated into relational form Advantage: mature database systems Disadvantages: overhead of translating data and queries

©Silberschatz, Korth and Sudarshan10.50Database System Concepts - 5 th Edition, Aug 22, Storage of XML in Relational Databases Alternatives: String Representation Tree Representation Map to relations

©Silberschatz, Korth and Sudarshan10.51Database System Concepts - 5 th Edition, Aug 22, String Representation Store each child of top level element as a string field of a tuple in a relational database Use a single relation to store all elements, or Use a separate relation for each top-level element type E.g. account, customer, depositor relations –Each with a string-valued attribute to store the element Indexing: Store values of subelements/attributes to be indexed as extra fields of the relation, and build indices on these fields E.g. customer_name or account_number Some database systems support function indices, which use the result of a function as the key value. The function should return the value of the required subelement/attribute

©Silberschatz, Korth and Sudarshan10.52Database System Concepts - 5 th Edition, Aug 22, String Representation (Cont.) Benefits: Can store any XML data even without DTD As long as the top-level element in a document has a large number of children, strings are small compared to full document Allows fast access to individual elements. Drawback: Need to parse strings to access values inside the elements Parsing is slow.

©Silberschatz, Korth and Sudarshan10.53Database System Concepts - 5 th Edition, Aug 22, Tree Representation Tree representation: model XML data as tree and store using relations nodes(id, type, label, value) child (child_id, parent_id) Each element/attribute is given a unique identifier Type indicates element/attribute Label specifies the tag name of the element/name of attribute Value is the text value of the element/attribute The relation child notes the parent-child relationships in the tree Can add an extra attribute to child to record ordering of children bank (id:1) customer (id:2) account (id: 5) customer_name (id: 3) account_number (id: 7)

©Silberschatz, Korth and Sudarshan10.54Database System Concepts - 5 th Edition, Aug 22, Tree Representation (Cont.) Benefit: Can store any XML data, even without DTD Drawbacks: Data is broken up into too many pieces, increasing space overheads Even simple queries require a large number of joins, which can be slow

©Silberschatz, Korth and Sudarshan10.55Database System Concepts - 5 th Edition, Aug 22, Mapping XML Data to Relations Relation created for each element type whose schema is known: An id attribute to store a unique id for each element A relation attribute corresponding to each element attribute A parent_id attribute to keep track of parent element As in the tree representation Position information (i th child) can be stored too All subelements that occur only once can become relation attributes For text-valued subelements, store the text as attribute value For complex subelements, can store the id of the subelement Subelements that can occur multiple times represented in a separate table Similar to handling of multivalued attributes when converting ER diagrams to tables

©Silberschatz, Korth and Sudarshan10.56Database System Concepts - 5 th Edition, Aug 22, Storing XML Data in Relational Systems Publishing: process of converting relational data to an XML format Shredding: process of converting an XML document into a set of tuples to be inserted into one or more relations XML-enabled database systems support automated publishing and shredding Some systems offer native storage of XML data using the xml data type. Special internal data structures and indices are used for efficiency

©Silberschatz, Korth and Sudarshan10.57Database System Concepts - 5 th Edition, Aug 22, SQL/XML New standard SQL extension that allows creation of nested XML output Each output tuple is mapped to an XML element row A-101 Downtown 500 …. more rows if there are more output tuples …

©Silberschatz, Korth and Sudarshan10.58Database System Concepts - 5 th Edition, Aug 22, SQL Extensions xmlelement creates XML elements xmlattributes creates attributes select xmlelement (name account, xmlattributes (account_number as account_number), xmlelement (name branch_name, branch_name), xmlelement (name balance, balance)) from account xmlforest(attr1, attr2,..) creates a sequence (forest) of one or more elements, with tag names same as the SQL attribute name xmlagg: aggregate function creates a forest from elements in group select xmlelement (name branch, branch_name, xmlagg(xmlforest(account_number) order by account_number) from account group by branch_name

©Silberschatz, Korth and Sudarshan10.59Database System Concepts - 5 th Edition, Aug 22, XML Application: Web Services The Simple Object Access Protocol (SOAP) standard: Invocation of procedures across applications with distinct databases XML used to represent procedure input and output A Web service is a site providing a collection of SOAP procedures Described using the Web Services Description Language (WSDL) Directories of Web services are described using the Universal Description, Discovery, and Integration (UDDI) standard