Hướng dẫn học Microsoft SQL Server 2008 part 52 ppsx

*/ Thequerymethod takes an XQuery expression that can be customized to locate and retrieve specific nodes matching a given condition: SELECT ItemData.query’/Order/Item[@ItemNumber="D001"

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</Orders>

*/

The following example demonstrates how to generate elements with multiple namespaces:

;WITH XMLNAMESPACES(

‘http://www.sqlserverbible.com/customers’ AS cust,

‘http://www.sqlserverbible.com/orders’ AS ord )

SELECT OrderNumber AS ‘ord:OrderNumber’, CustomerID AS ‘cust:CustomerID’

FROM OrderHeader WHERE OrderID = 1 FOR XML PATH(’’),ROOT(’Orders’) /*

<Orders xmlns:ord="http://www.sqlserverbible.com/orders"

xmlns:cust="http://www.sqlserverbible.com/customers">

<ord:OrderNumber>SO101</ord:OrderNumber>

<cust:CustomerID>1</cust:CustomerID>

</Orders>

*/

An element can be associated with a namespace by specifying a colonized name as the element name

(a name that contains a namespace name and element name separated by a colon)

WITH XMLNAMESPACESoffers a very easy way to generate XML output with namespace declarations

XML documents can be created with one or more namespace declarations

Understanding XQuery and FLWOR operations

XQuery is a W3C recommended language created for querying XML documents In a simplified sense,

one could say that ‘‘XQuery is to XML what SQL is to a relational database.’’

Thequery()method of theXMLdata type implements a subset of XQuery specifications and provides

a very extensive set of functionalities that enables performing a number of interesting operations on XML

documents

Simple queries

The basic usage of thequery()method is to retrieve one or more XML nodes from the given XML

document The result of thequery()method is always anXMLdata type value:

SELECT ItemData.query(’/Order/Item’) FROM OrderXML

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*/

Thequery()method takes an XQuery expression that can be customized to locate and retrieve specific

nodes matching a given condition:

SELECT

ItemData.query(’/Order/Item[@ItemNumber="D001"]’)

FROM OrderXML

/*

*/

The result of thequery()method can be used as input for other operations, such as the example

given here:

SELECT

OrderID,

ItemData.query(’

count(/Order/Item)

‘).value(’.’,’INT’) AS LineCount

FROM OrderXML

/*

OrderID LineCount

*/

This example used the XQuerycount()method to retrieve the number ofItemelements in each row

Thequery()method always returns anXMLdata type value; hence, thevalue()method is used to

retrieve anINTvalue from it

FLWOR operation

The true power of XQuery comes with theFLWORoperation, which is pronounced like ‘‘flower’’ and

stands forFOR LET WHERE ORDER BYandRETURN AFLWORoperation enables querying or

transform-ing XML documents It can be used either to extract specific information from an XML document or to

restructure the XML document and return a completely new XML value

A basicFLWORquery usingFORandRETURNis shown in the following example:

DECLARE @x XML

SELECT @x = ‘

<Items>

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</Items>’

SELECT

@x.query(’

for $item in Items/ItemNumber return $item ‘)

/*

*/

WHEREandORDER BYcan be specified to filter and sort the output:

DECLARE @x XML SELECT @x = ‘

<Items>

</Items>’

SELECT

@x.query(’

for $item in Items/ItemNumber where $item[ < "2000"]

order by $item return $item ‘)

/*

*/

TheWHEREcondition in the preceding XQuery expression filters the nodes forItemNumberless than

2000 TheORDER BYclause then orders the nodes byItemNumber

AFLWORoperation can be used to completely restructure an XML document, as demonstrated here:

<Item ItemNumber="Z001" Quantity="1" Price="200" />’

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@x.query(’

for $item in Item

return

{data($item/@ItemNumber)}

</ItemNumber> ‘) /*

*/

The preceding example transforms theItemNumberattributes to elements and produces a completely

different XML document Complex FLWORoperations can be applied on an XML document to achieve

very complex transformation requirements

What’s new for XQuery in SQL Server 2008

SQL Server 2008 adds support for theletclause inFLWORoperations Theletclause allows declaring

and using inline variables within the XQuery expression used in aFLWORquery:

DECLARE @x XML

SELECT @x = ‘

<Item ItemNumber="Z001" Quantity="3" Price="200" />’

SELECT

@x.query(’

for $item in Item

let $itm := $item/@ItemNumber

let $tot := $item/@Quantity * $item/@Price

return

<Item>

</Item>

‘)

/*

<Item>

</Item>

<Item>

</Item>

*/

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The XQuery expression in the preceding example declares two inline variables using theletclause:

$itmand$tot These variables are initialized within the XQuery expression and used to generate

cus-tom XML nodes

Theletclause reduces the complexity ofFLWORoperations by enabling complex expressions to be

translated into variables

Understanding XQuery Functions

The XQuery specification has defined a number of XQuery functions, many of which have been

implemented by SQL Server This section briefly examines the XQuery functions supported by SQL

Server 2008

String functions

String functions are one of the most commonly used set of functions in any language SQL Server 2008

supports the following XQuery string functions:

■ string()

■ concat()

■ substring()

■ contains()

■ string-length()

Thestring()function is a data accessor function that returns the string value from an element or

attribute Theconcat()function joins two string values, and thesubstring()function returns

a substring from a given value, starting at a specified position and having a specified length The

string-length()function returns the length of a given string value Finally, thecontains()

function accepts two string values and returns true if the second value is a substring of the first value

Numeric and aggregate functions

SQL Server 2008 supports the following XQuery aggregate functions These functions can be used to

perform aggregate operations over the nodes of an XML document:

■ min()

■ max()

■ count()

■ sum()

■ avg()

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Support for the following XQuery numeric functions is implemented in SQL Server 2008:

■ ceiling()

■ floor()

■ round()

The functionality exposed by each of these functions is self-explanatory All these functions exist in

T-SQL as well, and the T-T-SQL versions of these functions provide the same functionality as the XQuery

versions (except that the T-SQL function is applied on relational rows, whereas the XQuery version is

applied on XML nodes)

Other functions

SQL Server 2008 supports a few more XQuery functions that may be less commonly used, but are still

worth a mention here:

■ Data accessor function:data()

■ Boolean functions:not(),true(), andfalse()

■ Sequence functions:empty(),distinct-values(), andid()

■ Node functions:number(),local-name(), andnamespace-uri()

■ Context functions:last()andposition()

■ QName functions:expanded-QName(),local-name-from-QName()and

namespace-uri-from-QName

■ SQL Server XQuery extension functions:sql:variable()andsql:column()

The XQuery implementation of SQL Server 2008 still does not support user-defined functions (UDFs),

recommended by the XQuery specification Future versions of SQL Server might add support for

user-defined functions within XQuery expressions, and the addition of XQuery UDFs will provide more

programming power to the T-SQL developer

Performing XML Data Modification

Themodify()method of theXMLdata type can be used to perform DML operations on XML

docu-ments It allows performing insert, update, and delete operations on XML variables or columns

Theinsert,replace value of, anddeleteinstructions are used with themodify()method to

performINSERT,UPDATE, andDELETEoperations on XML documents Each of these operations has

its own syntax and is briefly explained in the following sections

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Insert operation

A new element or attribute can be inserted into an XML document by using themodify()method

with theinsertcommand A basic example demonstrating aninsertoperation is given here:

DECLARE @x XML SELECT @x = ‘<SalesOrder OrderNumber="SO101"/>’

DECLARE @CustomerID INT SELECT @CustomerID = 1 SET @x.modify(’

insert element CustomerID {sql:variable("@CustomerID")}

as last into (SalesOrder)[1]

‘)

SELECT @x /*

</SalesOrder>

*/

Update operation

Themodify()method can be used withreplace value ofcommand to modify the value of

ele-ments or attributes The following example changes the value of theCustomerIDelement:

</SalesOrder>’

DECLARE @CustomerID INT SELECT @CustomerID = 2

SET @x.modify(’

replace value of (SalesOrder/CustomerID/text())[1]

with sql:variable("@CustomerID")

‘)

SELECT @x

/*

</SalesOrder>

*/

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Delete operation

Themodify()method can be used with thedeleteinstruction to remove an element or attribute

from a given XML document The following example deletes theCustomerIDelement from the XML

document:

DECLARE @x XML

SELECT @x = ‘

</SalesOrder>’

SET @x.modify(’

delete (SalesOrder/CustomerID)[1]

‘)

SELECT @x

/*

*/

What’s new for XML DML operations in SQL Server 2008

SQL Server 2005’s implementation of themodify()method does not support XML variables with

theinsertinstruction With SQL Server 2005, it is not possible to insert an XML value into an

XML document A possible workaround is to cast the XML value toVARCHAR/NVARCHARand

per-form an insert operation However, in such a case, SQL Server will encode the XML tags within the

VARCHAR/NVARCHARvalue, which may not be the desired result most of the time

SQL Server 2008 enhanced themodify()method to support XML variables with theinsert

com-mand The following code snippet shows an example:

DECLARE @doc XML, @val XML

SELECT @doc = ‘

</SalesOrder>’

SELECT @val = ‘

<Items>

</Items>’

SET @doc.modify(’

insert sql:variable("@val")

as last into (SalesOrder)[1]

‘)

SELECT @doc

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<Items>

</Items>

</SalesOrder>

*/

Note that the syntax of theinsertcommand is slightly different when an XML value is being inserted

Handling Namespaces

XML uses namespaces to disambiguate elements and attributes If the XML document contains

name-space declarations, then the XQuery expressions for querying or modifying the XML document should

also contain the required namespace declarations to identify and access the correct XML nodes

TheWITH NAMESPACESdirective can be used to declare the XML namespaces and refer to them in the

XQuery expressions following the declaration:

<SalesOrder xmlns="http://www.sqlserverbible.com/order"

xmlns:cust="http://www.sqlserverbible.com/customer">

<cust:CustomerID>10001</cust:CustomerID>

</SalesOrder>’

;WITH XMLNAMESPACES(

DEFAULT ‘http://www.sqlserverbible.com/order’,

‘http://www.sqlserverbible.com/customer’ AS cust )

SELECT

@x.value(’(SalesOrder/OrderID)[1]’,’INT’) AS OrderID,

@x.value(’(SalesOrder/cust:CustomerID)[1]’,’INT’) AS CustomerID

/*

OrderID CustomerID

*/

TheWITH XMLNAMESPACESdirective simplifies namespace declaration, as the namespaces can be

declared once and then reused in the XQuery expression that follows the declaration

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Shredding XML Using OPENXML()

OPENXML(), released along with SQL Server 2000, was the first XML shredding function added to SQL

Server.OPENXML()is very powerful and is the only option available to shred XML documents (from

T-SQL) in SQL Server 2000 SQL Server 2005 added support for XQuery, which is a better choice over

OPENXML() in most cases SQL Server 2008 enhanced the XQuery functionalities further by adding

support for ‘‘let’’ clause in FLWOR operations

The following example shows a basicOPENXML()function call:

DECLARE @hdoc INT

DECLARE @xml VARCHAR(MAX)

SET @xml =’

<Items>

</Items>

</SalesOrder>’

Step 1: initialize XML Document Handle

EXEC sp_xml_preparedocument @hdoc OUTPUT, @xml

Step 2: Call OPENXML()

SELECT * FROM OPENXML(@hdoc, ‘/SalesOrder/Items/Item’)

WITH (

OrderNumber CHAR(5) ‘ / /@OrderNumber’,

ItemNumber CHAR(4) ‘@ItemNumber’,

Quantity INT ‘@Quantity’,

Price MONEY ‘@Price’

)

Step 3: Free document handle

exec sp_xml_removedocument @hdoc

/*

OrderNumber ItemNumber Quantity Price

- - -

*/

As is apparent from the preceding example, a call toOPENXML()is always a three-step process Before

the function can be called, a document handle for the current XML document should be created and

initialized This handle should be passed to theOPENXML()function call as an argument Finally, the

handle has to be released to free system resources used for the operation

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