Скачать презентацию Chapter 10 XML Database System Concepts Silberschatz Korth
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Chapter 10: XML Database System Concepts ©Silberschatz, Korth and Sudarshan See www. db-book. com for conditions on re-use
XML n Structure of XML Data n XML Document Schema n Querying and Transformation n Application Program Interfaces to XML n Storage of XML Data n XML Applications Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 2 ©Silberschatz, Korth and Sudarshan
Introduction n XML: Extensible Markup Language n Defined by the WWW Consortium (W 3 C) n Derived from SGML (Standard Generalized Markup Language), but simpler to use than SGML n Documents have tags giving extra information about sections of the document l E. g.
XML Introduction (Cont. ) n The ability to specify new tags, and to create nested tag structures make XML a great way to exchange data, not just documents. l Much of the use of XML has been in data exchange applications, not as a replacement for HTML n Tags make data (relatively) self-documenting l E. g.
XML: Motivation n Data interchange is critical in today’s networked world l Examples: 4 Banking: 4 Order funds transfer processing (especially inter-company orders) 4 Scientific data – Chemistry: Chem. ML, … – Genetics: l BSML (Bio-Sequence Markup Language), … Paper flow of information between organizations is being replaced by electronic flow of information n Each application area has its own set of standards for representing information n XML has become the basis for all new generation data interchange formats Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 5 ©Silberschatz, Korth and Sudarshan
XML Motivation (Cont. ) n Earlier generation formats were based on plain text with line headers indicating the meaning of fields l Similar in concept to email headers l Does not allow for nested structures, no standard “type” language l Tied too closely to low level document structure (lines, spaces, etc) n Each XML based standard defines what are valid elements, using l XML type specification languages to specify the syntax 4 DTD 4 XML l (Document Type Descriptors) Schema Plus textual descriptions of the semantics n XML allows new tags to be defined as required l However, this may be constrained by DTDs n A wide variety of tools is available for parsing, browsing and querying XML documents/data Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 6 ©Silberschatz, Korth and Sudarshan
Comparison with Relational Data n Inefficient: tags, which in effect represent schema information, are repeated n Better than relational tuples as a data-exchange format l Unlike relational tuples, XML data is self-documenting due to presence of tags l Non-rigid format: tags can be added l Allows nested structures l Wide acceptance, not only in database systems, but also in browsers, tools, and applications Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 7 ©Silberschatz, Korth and Sudarshan
Structure of XML Data n Tag: label for a section of data n Element: section of data beginning with
Example of Nested Elements
Motivation for Nesting n Nesting of data is useful in data transfer l Example: elements representing customer_id, customer_name, and address nested within an order element n Nesting is not supported, or discouraged, in relational databases l With multiple orders, customer name and address are stored redundantly l normalization replaces nested structures in each order by foreign key into table storing customer name and address information l Nesting is supported in object-relational databases n But nesting is appropriate when transferring data l External application does not have direct access to data referenced by a foreign key Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 10 ©Silberschatz, Korth and Sudarshan
Structure of XML Data (Cont. ) n Mixture of text with sub-elements is legal in XML. Example:
Attributes n Elements can have attributes
Attributes vs. Subelements n Distinction between subelement and attribute l In the context of documents, attributes are part of markup, while subelement contents are part of the basic document contents l In the context of data representation, the difference is unclear and may be confusing 4 Same information can be represented in two ways –
Namespaces n XML data has to be exchanged between organizations n Same tag name may have different meaning in different organizations, causing confusion on exchanged documents n Specifying a unique string as an element name avoids confusion n Better solution: use unique-name: element-name n Avoid using long unique names all over document by using XML Namespaces
More on XML Syntax n Elements without subelements or text content can be abbreviated by ending the start tag with a /> and deleting the end tag l
XML Document Schema n Database schemas constrain what information can be stored, and the data types of stored values n XML documents are not required to have an associated schema n However, schemas are very important for XML data exchange l Otherwise, a site cannot automatically interpret data received from another site n Two mechanisms for specifying XML schema l Document Type Definition (DTD) 4 Widely l used XML Schema 4 Newer, increasing use Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 16 ©Silberschatz, Korth and Sudarshan
Document Type Definition (DTD) n The type of an XML document can be specified using a DTD n DTD constraints structure of XML data l What elements can occur l What attributes can/must an element have l What subelements can/must occur inside each element, and how many times. n DTD does not constrain data types l All values represented as strings in XML n DTD syntax l l Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 17 ©Silberschatz, Korth and Sudarshan
Element Specification in DTD n Subelements can be specified as l names of elements, or l #PCDATA (parsed character data), i. e. , character strings l EMPTY (no subelements) or ANY (anything can be a subelement) n Example n Subelement specification may have regular expressions 4 Notation: – “|” - alternatives – “+” - 1 or more occurrences – “*” - 0 or more occurrences Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 18 ©Silberschatz, Korth and Sudarshan
Bank DTD ]> Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 19 ©Silberschatz, Korth and Sudarshan
Attribute Specification in DTD n Attribute specification : for each attribute l Name l Type of attribute 4 CDATA 4 ID (identifier) or IDREF (ID reference) or IDREFS (multiple IDREFs) – more on this later l Whether 4 mandatory 4 has 4 or (#REQUIRED) a default value (value), neither (#IMPLIED) n Examples l l Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 20 ©Silberschatz, Korth and Sudarshan
IDs and IDREFs n An element can have at most one attribute of type ID n The ID attribute value of each element in an XML document must be distinct l Thus the ID attribute value is an object identifier n An attribute of type IDREF must contain the ID value of an element in the same document n 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 Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 21 ©Silberschatz, Korth and Sudarshan
Bank DTD with Attributes n Bank DTD with ID and IDREF attribute types. … declarations for branch, balance, customer_name, customer_street and customer_city ]> Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 22 ©Silberschatz, Korth and Sudarshan
XML data with ID and IDREF attributes
Limitations of DTDs n No typing of text elements and attributes l All values are strings, no integers, reals, etc. n Difficult to specify unordered sets of subelements l Order is usually irrelevant in databases (unlike in the documentlayout environment from which XML evolved) l (A | B)* allows specification of an unordered set, but 4 Cannot ensure that each of A and B occurs only once n IDs and IDREFs are untyped l The owners attribute of an account may contain a reference to another account, which is meaningless 4 owners attribute should ideally be constrained to refer to customer elements Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 24 ©Silberschatz, Korth and Sudarshan
XML Schema n XML Schema is a more sophisticated schema language which addresses the drawbacks of DTDs. Supports l Typing of values 4 E. g. integer, string, etc 4 Also, constraints on min/max values l User-defined, complex types l Many more features, including 4 uniqueness and foreign key constraints, inheritance n XML Schema is itself specified in XML syntax, unlike DTDs l More-standard representation, but verbose n XML Scheme is integrated with namespaces n BUT: XML Schema is significantly more complicated than DTDs. Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 25 ©Silberschatz, Korth and Sudarshan
XML Schema Version of Bank DTD
XML Schema Version of Bank DTD n Choice of “xs: ” was ours -- any other namespace prefix could be chosen n Element “bank” has type “Bank. Type”, which is defined separately l xs: complex. Type is used later to create the named complex type “Bank. Type” n Element “account” has its type defined in-line Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 27 ©Silberschatz, Korth and Sudarshan
More features of XML Schema n Attributes specified by xs: attribute tag: l
Querying and Transforming XML Data n Translation of information from one XML schema to another n Querying on XML data n Above two are closely related, and handled by the same tools n Standard XML querying/translation languages l XPath 4 Simple l language consisting of path expressions XSLT 4 Simple language designed for translation from XML to XML and XML to HTML l XQuery 4 An XML query language with a rich set of features Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 29 ©Silberschatz, Korth and Sudarshan
Tree Model of XML Data n Query and transformation languages are based on a tree model of XML data n An XML document is modeled as a tree, with nodes corresponding to elements and attributes l Element nodes have child nodes, which can be attributes or subelements l Text in an element is modeled as a text node child of the element l Children of a node are ordered according to their order in the XML document l Element and attribute nodes (except for the root node) have a single parent, which is an element node l The root node has a single child, which is the root element of the document Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 30 ©Silberschatz, Korth and Sudarshan
XPath n XPath is used to address (select) parts of documents using path expressions n A path expression is a sequence of steps separated by “/” l Think of file names in a directory hierarchy n Result of path expression: set of values that along with their containing elements/attributes match the specified path n E. g. /bank-2/customer_name evaluated on the bank-2 data we saw earlier returns
XPath (Cont. ) n The initial “/” denotes root of the document (above the top-level tag) n Path expressions are evaluated left to right l Each step operates on the set of instances produced by the previous step n Selection predicates may follow any step in a path, in [ ] l E. g. /bank-2/account[balance > 400] 4 returns account elements with a balance value greater than 400 4 /bank-2/account[balance] returns account elements containing a balance subelement n Attributes are accessed using “@” l E. g. /bank-2/account[balance > 400]/@account_number 4 returns l the account numbers of accounts with balance > 400 IDREF attributes are not dereferenced automatically (more on this later) Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 32 ©Silberschatz, Korth and Sudarshan
Functions in 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 4 E. g. /bank-2/account[count(. /customer) > 2] – Returns accounts with > 2 customers l Also function for testing position (1, 2, . . ) of node w. r. t. siblings n Boolean connectives and or and function not() can be used in predicates n IDREFs can be referenced using function id() l id() can also be applied to sets of references such as IDREFS and even to strings containing multiple references separated by blanks l E. g. /bank-2/account/id(@owner) 4 returns all customers referred to from the owners attribute of account elements. l Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 33 ©Silberschatz, Korth and Sudarshan
More XPath Features n Operator “|” used to implement union E. g. /bank-2/account/id(@owner) | /bank-2/loan/id(@borrower) 4 Gives customers with either accounts or loans 4 However, “|” cannot be nested inside other operators. n “//” can be used to skip multiple levels of nodes l E. g. /bank-2//customer_name 4 finds any customer_name element anywhere under the /bank-2 element, regardless of the element in which it is contained. n 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 l “//”, described above, is a short from for specifying “all descendants” l “. . ” specifies the parent. n doc(name) returns the root of a named document l Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 34 ©Silberschatz, Korth and Sudarshan
XQuery n XQuery is a general purpose query language for XML data n Currently being standardized by the World Wide Web Consortium (W 3 C) l 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. n XQuery is derived from the Quilt query language, which itself borrows from SQL, XQL and XML-QL n 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 Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 35 ©Silberschatz, Korth and Sudarshan
FLWOR Syntax in XQuery n For clause uses XPath expressions, and variable in for clause ranges over values in the set returned by XPath n Simple FLWOR expression in XQuery l find all accounts with balance > 400, with each result enclosed in an
Joins n 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
Nested Queries n The following query converts data from the flat structure for bank (fig 10. 1) information into the nested structure used in bank-1(fig. 10. 4)
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
Functions and Other XQuery Features n 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 } n Types are optional for function parameters and return values n The * (as in decimal*) indicates a sequence of values of that type n Universal and existential quantification in where clause predicates l some $e in path satisfies P l every $e in path satisfies P n XQuery also supports If-then-else clauses Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 40 ©Silberschatz, Korth and Sudarshan
XSLT n A stylesheet stores formatting options for a document, usually separately from document l E. g. an HTML style sheet may specify font colors and sizes for headings, etc. n The XML Stylesheet Language (XSL) was originally designed for generating HTML from XML n XSLT is a general-purpose transformation language l Can translate XML to XML, and XML to HTML n XSLT transformations are expressed using rules called templates l Templates combine selection using XPath with construction of results Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 41 ©Silberschatz, Korth and Sudarshan
XSLT Templates n Example of XSLT template with match and select part
Creating XML Output n Any text or tag in the XSL stylesheet that is not in the xsl namespace is output as is n E. g. to wrap results in new XML elements.
Creating XML Output (Cont. ) n Note: Cannot directly insert a xsl: value-of tag inside another tag E. g. cannot create an attribute for
Structural Recursion n Template action can apply templates recursively to the contents of a matched element
Joins in XSLT keys allow elements to be looked up (indexed) by values of subelements or attributes l Keys must be declared (with a name) and, the key() function can then be used for lookup. E. g.
Sorting in XSLT n Using an xsl: sort directive inside a template causes all elements matching the template to be sorted l Sorting is done before applying other templates
Application Program Interface n There are two standard application program interfaces to XML data: l SAX (Simple API for XML) 4 Based on parser model, user provides event handlers for parsing events – E. g. start of element, end of element – Not suitable for database applications l DOM (Document Object Model) 4 XML data is parsed into a tree representation 4 Variety 4 E. g. : 4 Also of functions provided for traversing the DOM tree Java DOM API provides Node class with methods get. Parent. Node( ), get. First. Child( ), get. Next. Sibling( ) get. Attribute( ), get. Data( ) (for text node) get. Elements. By. Tag. Name( ), … provides functions for updating DOM tree Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 48 ©Silberschatz, Korth and Sudarshan
Storage of XML Data n XML data can be stored in l Non-relational data stores 4 Flat files – Natural for storing XML – But has all problems discussed in Chapter 1 (no concurrency, no recovery, …) 4 XML database – Database built specifically for storing XML data, supporting DOM model and declarative querying – Currently no commercial-grade systems l Relational databases 4 Data must be translated into relational form 4 Advantage: mature database systems 4 Disadvantages: Database System Concepts - 5 th Edition, Aug 22, 2005. overhead of translating data and queries 10. 49 ©Silberschatz, Korth and Sudarshan
Storage of XML in Relational Databases n Alternatives: l String Representation l Tree Representation l Map to relations Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 50 ©Silberschatz, Korth and Sudarshan
String Representation n Store each top level element as a string field of a tuple in a relational database l Use a single relation to store all elements, or l Use a separate relation for each top-level element type 4 E. g. account, customer, depositor relations – Each with a string-valued attribute to store the element n Indexing: l Store values of subelements/attributes to be indexed as extra fields of the relation, and build indices on these fields 4 E. g. l customer_name or account_number Some database systems support function indices, which use the result of a function as the key value. 4 The function should return the value of the required subelement/attribute Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 51 ©Silberschatz, Korth and Sudarshan
String Representation (Cont. ) n Benefits: l Can store any XML data even without DTD l As long as there are many top-level elements in a document, strings are small compared to full document 4 Allows fast access to individual elements. n Drawback: Need to parse strings to access values inside the elements l Parsing is slow. Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 52 ©Silberschatz, Korth and Sudarshan
Tree Representation n Tree representation: model XML data as tree and store using relations nodes(id, type, label, value) child (child_id, parent_id) bank (id: 1) customer (id: 2) account (id: 5) customer_name (id: 3) account_number (id: 7) n Each element/attribute is given a unique identifier n Type indicates element/attribute n Label specifies the tag name of the element/name of attribute n Value is the text value of the element/attribute n The relation child notes the parent-child relationships in the tree l Can add an extra attribute to child to record ordering of children Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 53 ©Silberschatz, Korth and Sudarshan
Tree Representation (Cont. ) n Benefit: Can store any XML data, even without DTD n Drawbacks: l Data is broken up into too many pieces, increasing space overheads l Even simple queries require a large number of joins, which can be slow Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 54 ©Silberschatz, Korth and Sudarshan
Mapping XML Data to Relations n Relation created for each element type whose schema is known: l An id attribute to store a unique id for each element l A relation attribute corresponding to each element attribute l A parent_id attribute to keep track of parent element 4 As in the tree representation 4 Position information (ith child) can be store too n All subelements that occur only once can become relation attributes l For text-valued subelements, store the text as attribute value l For complex subelements, can store the id of the subelement n Subelements that can occur multiple times represented in a separate table l Similar to handling of multivalued attributes when converting ER diagrams to tables Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 55 ©Silberschatz, Korth and Sudarshan
Storing XML Data in Relational Systems n Publishing: process of converting relational data to an XML format n Shredding: process of converting an XML document into a set of tuples to be inserted into one or more relations n XML-enabled database systems support automated publishing and shredding n Some systems offer native storage of XML data using the xml data type. Special internal data structures and indices are used for efficiency Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 56 ©Silberschatz, Korth and Sudarshan
SQL/XML n New standard SQL extension that allows creation of nested XML output l Each output tuple is mapped to an XML element row
SQL Extensions n xmlelement creates XML elements n 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 Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 58 ©Silberschatz, Korth and Sudarshan
Web Services n The Simple Object Access Protocol (SOAP) standard: l Invocation of procedures across applications with distinct databases l XML used to represent procedure input and output n A Web service is a site providing a collection of SOAP procedures l Described using the Web Services Description Language (WSDL) l Directories of Web services are described using the Universal Description, Discovery, and Integration (UDDI) standard Database System Concepts - 5 th Edition, Aug 22, 2005. 10. 59 ©Silberschatz, Korth and Sudarshan
