EMERGING TECHNOLOGIES

Transcription

1 EMERGING TECHNOLOGIES XML (Part 3): XQuery

2 Outline 1. Introduction 2. Structure of XML data 3. XML Document Schema 3.1. Document Type Definition (DTD) 3.2. XMLSchema 4. Data Model for XML documents. 5. XML Query Languages 5.1. XPath 5.2. XQuery

3 5.2 XQuery (1) Introduction XQuery is a general purpose query language for XML data. XQuery permits the specification of more general queries on one or more documents. The result of a query in XQuery is an XML document. XQuery is the W3C standard query language for XML data. Full Specification can be found in the site of the World Wide Web Consortium (W3C):

4 5.2 XQuery (2) Syntax The typical form of a query in XQuery is known as a FLWR (pronounced "flower") expression. It stands for the four main clauses of XQuery and has the following form: for <variable bindings to individual nodes (elements)> let <variable bindings to collections of nodes (elements)> where <qualifier conditions> return <query result specification>

5 5.2 XQuery (3) Syntax for <variable bindings to individual nodes (elements)> let <variable bindings to collections of nodes (elements)> where <qualifier conditions> return <query result specification> The for clause gives a series of variables that range over the results of XPath expressions. The let clause simply allows complicated expressions to be assigned to variable names for simplicity of presentation. The where clause performs additional tests on the values of the variables. The return clause allows the construction of results in XML.

6 5.2 XQuery (4) Syntax Comparing XQuery to SQL: for SQL from let allows temporary variables, and has no equivalent in SQL where SQL where return SQL select When more than one variable is specified in the for clause, the results include the Cartesian product of all the possible values the variables can take.

7 5.2 XQuery (5) Example XML Document <bank-2> <account account-number= A-401 owners= C100 C102 > <branch-name> Downtown </branch-name> <balance>500</balance> </account> <account account-number= A-402 owners= C102 C101 > <branch-name> Perryridge </branch-name> <balance>900</balance> </account> (continues in the next slide)

8 5.2 XQuery (6) Example XML Document (cont., from previous slide) <customer customer-id= C100 accounts= A-401 > <customer-name>joe</customer-name> <customer-street>monroe</customer-street> <customer-city>madison</customer-city> </customer> <customer customer-id= C101 accounts= A-402 > <customer-name>lisa</customer-name> <customer-street>mountain</customer-street> <customer-city>murray Hill</customer-city> </customer> <customer customer-id= C102 accounts= A-401 A-402 > <customer-name> Mary</customer-name> <customer-street> Erin</customer-street> <customer-city> Newark </customer-city> </customer> </bank-2>

9 5.2 XQuery (7) The DTD of the Example XML Document <!DOCTYPE bank-2[ <!ELEMENT bank-2 (account*, customer*)> <!ELEMENT account (branch-name, balance)> <!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-name, balance, customer-name, customer-street and customer-city ]>

10 5.2 XQuery (8) Syntax Example Simple FLWR expression in XQuery: find all accounts with balance > 400, with each result enclosed in an <account-number>... </account-number> tag for $x in /bank-2/account let $acctno := where $x/balance > 400 return <account-number>$acctno</account-number> 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 <account-number> </account-number> However, the let clause simplifies complex queries.

11 5.2 XQuery (9) Syntax Example (cont.) The previous answer produces the following result when applied to the XML document bank-2. <account-number>a-401</account-number> <account-number>a-402</account-number>

12 5.2 XQuery (10) Syntax Path expressions are used to bind variables in the for clause, but can also be used in other places. E.g. path expressions can be used in let clause, to bind variables to results of path expressions. Path expressions in XQuery may return a multiset, with repeated nodes. The function distinct( ) can be applied to a multiset, to return a set without duplication. The distinct function can be used even within a for clause.

13 5.2 XQuery (11) Syntax The function document(name) returns the root of named document. E.g. document( bank-2.xml )/bank-2/account. Aggregate functions such as sum( ) and count( ) can be applied to path expression results. XQuery does not support group by, but the same effect can be obtained by nested queries, with nested FLWR expressions within a result clause.

14 5.2 XQuery (12) Example XML document <bank> <account> <account-number>a-101</account-number> <branch-name>dowtown</branch-name> <balance> 500</balance> </account> <account> <account-number>a-102</account-number> <branch-name>perryridge</branch-name> <balance>400</balance> </account> <account> <account-number>a-201</account-number> <branch-name>brighton</branch-name> <balance>900</balance> </account> <customer> <customer-name>johnson</customer-name> <customer-street>alma</customer-street> <customer-city>palo Alto</customer-city> </customer>

15 5.2 XQuery (13) Example XML Document (cont.) <customer> <customer-name>hayes</customer-name> <customer-street>main</customer-street> <customer-city>harrison</customer-city> </customer> <depositor> <account-number>a-101</account-number> <customer-name>johnson</customer-name> </ depositor > <depositor> <account-number>a-201</account-number> <customer-name>johnson</customer-name> </ depositor > <depositor> <account-number>a-102</account-number> <customer-name>hayes</customer-name> </ depositor > </bank>

16 5.2 XQuery (14) 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 <cust-acct> $c $a </cust-acct> 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 <cust-acct> $c $a</cust-acct>

17 5.2 XQuery (15) Changing Nesting Structure Query FLWR expressions can be nested in the return clause, in order to generate element nestings that do not appear in the source document. Example: The following query converts data from the flat structure for bank information into the nested structure used in bank-1. <bank-1> for $c in /bank/customer return <customer> $c/* for $d in /bank/depositor[customer-name = $c/customer-name], $a in /bank/account[accountnumber = $d/account-number] return $a </customer> </bank-1>

18 5.2 XQuery (16) Other features $c/text() gives text content of an element without any subelements/tags XQuery path expressions support the id( ) function of XPath. Can be applied to a set of IDREFs to get a set of results.

19 5.2 XQuery (17) Sorting Sortby clause can be used at the end of any expression. Example: to return customers sorted by name for $c in /bank/customer return <customer> $c/* </customer> sortby(customer-name) To sort in descending order, we can use sortby(name descending)

20 5.2 XQuery (18) Sorting Can sort at multiple levels of nesting (sort by customer-name, and by account-number within each customer) Example <bank-1> for $c in /bank/customer return <customer> $c/* for $d in /bank/depositor[customer-name = $c/customer-name], $a in /bank/account[account-number = $d/account-number] return <account> $a/* </account> sortby(account-number) </customer> sortby(customer-name) </bank-1>

21 5.2 XQuery (19) Functions User defined functions with the type system of XMLSchema function balances(xsd:string $c) returns list(xsd:numeric) { 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

22 5.2 XQuery (20) Other features 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