Hướng dẫn học Microsoft SQL Server 2008 part 49 docx

The XML data type implements a limited subset of the XQuery specification and a T-SQL query can use XQuery to retrieve information from XML columns or variables.. Static type checking is

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creating tables with XML columns, and allows declaring XML variables and using them as parameters

and return values

XQuery is a W3C-recommended language created to query and format XML documents XQuery can

be used to query XML documents just like a SQL query is used to retrieve information from relational

tables The XML data type implements a limited subset of the XQuery specification and a T-SQL query

can use XQuery to retrieve information from XML columns or variables

XQuery is built into the Relational Engine of SQL Server, and the Query Optimizer can build query

plans that contain relational query operations as well as XQuery operations Results of XQuery

opera-tions can be joined with relational data, or relational data can be joined with XQuery results SQL Server

supports creating special types of indexes on XML columns to optimize XQuery operations

XML Schema Definition (XSD) is another W3C-recommended language created for describing and

vali-dating XML documents XSD supports creating very powerful and complex validation rules that can be

applied to XML documents to verify that they are fully compliant with the business requirements The

XML data type supports XSD validation and is explained later in this chapter

The XML data type supports a number of methods, listed here:

■ modify()

Each of these methods is explained in detail later in this chapter

Typed and untyped XML

As mentioned earlier, support for XSD schema validation is implemented in SQL Server in the form

of XML schema collections An XML schema collection can be created from an XML schema definition

XML columns or variables can be bound to an XML schema collection An XML column or variable that

is bound to an XML schema collection is known as typed XML

When a typed XML value is modified, SQL Server validates the new value against the rules defined in

the XML schema collection The assignment or modification operation will succeed only if the new value

passes the validations defined in the XML schema collection

Typed XML has a number of advantages over untyped XML columns or variables The most important

benefit is that the validation constraints are always respected The content of a typed XML document is

always valid as per the schema with which it is associated

With typed XML, SQL Server has better knowledge of the XML document (structure, data types, and

so on) and can generate a more optimized query plan Because SQL Server has complete knowledge of

the data types of elements and attributes, storage of typed XML can be made significantly more compact

than untyped XML

Static type checking is possible with typed XML documents, and SQL Server can detect, at compile time,

if an XQuery expression on a typed XML document is mistyped Stored procedures or functions that

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accept typed XML parameters are protected from receiving invalid XML documents, as SQL Server will

perform implicit validation of the XML value against the schema collection before accepting the

parame-ter value

Creating and using XML columns

The XML data type can be used like other native SQL Server data types in most cases (Note that there

are exceptions, however For example, an XML column cannot be added as a column to a regular index

or used in a comparison operation.) A table can be created with one or more XML columns, or XML

columns can be added to an existing table.VARCHAR/NVARCHAR/VARBINARY/TEXT/NTEXTcolumns

can be altered to XML data type columns if all the existing values are well-formed XML values

Entire XML documents can be retrieved as part of aSELECTquery, or specific information can be

extracted from within the XML documents The following example shows aSELECTquery that selects a

column from a table and a value from the XML document stored in each row:

DECLARE @t TABLE (OrderID INT,OrderData XML )

INSERT INTO @t(OrderID, OrderData)

SELECT 1,

‘<CustomerNumber>1001</CustomerNumber>

<Items>

</Items>’

SELECT

OrderID,

OrderData.value(’CustomerNumber[1]’,’CHAR(4)’) AS CustomerNumber

FROM @t

/*

-

*/

The code might get a little more complex if the query needs to retrieve more than one element from the

XML document stored in each row Such a query needs to generate more than one row against each row

stored in the base table Thenodes()method of the XML data type can be used to obtain an

acces-sor to each element within the XML document The XML element collection returned by thenodes()

method can be joined with the base table using theCROSS APPLYoperator as shown in the following

example:

DECLARE @t TABLE (OrderID INT,OrderData XML )

INSERT INTO @t(OrderID, OrderData)

SELECT 1,

‘<CustomerNumber>1001</CustomerNumber>

<Items>

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</Items>’

SELECT OrderID, o.value(’@ItemNumber’,’CHAR(4)’) AS ItemNumber, o.value(’@Quantity’,’INT’) AS Quantity,

o.value(’@Price’,’MONEY’) AS Price FROM @t

CROSS APPLY OrderData.nodes(’/Items/Item’) x(o) /*

- - -

*/

The preceding examples use thevalue()method exposed by the XML data type XML data type

meth-ods are explained in detail later in this section

Declaring and using XML variables

Just like other SQL Server native data types, XML variables can be created and used in T-SQL batches,

stored procedures, functions, and so on The following example demonstrates a few different ways an

XML variable can be declared:

Declare an XML variable DECLARE @x XML

Declare a TYPED XML Variable DECLARE @x XML(CustomerSchema) Declare a TYPED XML DOCUMENT Variable DECLARE @x XML(DOCUMENT CustomerSchema) Declare a TYPED XML CONTENT variable DECLARE @x XML(CONTENT CustomerSchema)

The first example creates an untyped XML variable, and the second example creates a typed one The

third example creates aDOCUMENTtype variable, and the last one creates aCONTENTtype variable

DOCUMENTandCONTENTtypes are explained later in this chapter

There is a slight difference in the way that an XQuery expression needs to be written for an XML

variable versus an XML column While working with an XML variable, the query will always process

only one document at a time However, while working with an XML column, more than one XML

document may be processed in a single batch operation Because of this, theCROSS APPLY

oper-ator is required while running such a query on an XML column (as demonstrated in the previous

example)

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What follows is the version of the prior query that operates on an XML variable:

DECLARE @x XML

SELECT @x = ‘

<Items>

</Items>’

SELECT

o.value(’@ItemNumber’,’CHAR(4)’) AS ItemNumber,

o.value(’@Quantity’,’INT’) AS Quantity,

o.value(’@Price’,’MONEY’) AS Price

FROM @x.nodes(’/Items/Item’) x(o)

/*

ItemNumber Quantity Price

- -

*/

An XML variable may be initialized by a static XML string, from another XML orVARCHAR/NVARCHAR/

VARBINARYvariable, from the return value of a function, or from the result of aFOR XMLquery The

following example shows how to initialize an XML variable from the result of aFOR XMLquery:

DECLARE @x XML

SELECT @x = (

SELECT OrderID

FROM OrderHeader

FOR XML AUTO, TYPE)

XML variables can also be initialized from an XML file, as demonstrated later in the section ‘‘Loading

XML Documents from Disk Files.’’

Using XML parameters and return values

Typed and untyped XML parameters can be passed to a stored procedure asINPUTas well asOUTPUT

parameters XML parameters can be used as arguments as well as the return value of scalar functions or

in result columns of table-valued functions

When a function returns an XML data type value, XML data type methods can be directly called on the

return value, as shown in the following example:

Create a function that returns an XML value

CREATE FUNCTION GetOrderInfo(

@OrderID INT

) RETURNS XML

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AS BEGIN DECLARE @x XML SELECT @x = ( SELECT OrderID, CustomerID FROM OrderHeader

WHERE OrderID = @OrderID FOR XML PATH(’’),ROOT(’OrderInfo’)) RETURN @x

END GO Call the function and invoke the value() method SELECT dbo.GetOrderInfo(1).value(’(OrderInfo/CustomerID)[1]’,’INT’)

AS CustomerID /*

CustomerID -1

*/

Loading/querying XML documents from disk files

The capability to load XML documents from disk files is one of the very interesting XML features

available with SQL Server This is achieved by using theBULKrow set provider forOPENROWSET The

following example shows how to load the content of an XML file into an XML variable:

/*The sample code below assumes that a file named "items.xml"

exists in folder c:\temp with the following content

<Items>

</Items>

*/

DECLARE @xml XML SELECT

@xml = CAST(bulkcolumn AS XML) FROM OPENROWSET(BULK ‘C:\temp\items.xml’, SINGLE_BLOB) AS x SELECT

x.value(’@ItemNumber’,’CHAR(4)’) AS ItemNumber, x.value(’@Quantity’,’INT’) AS Quantity,

x.value(’@Price’,’MONEY’) AS Price FROM @xml.nodes(’/Items/Item’) i(x)

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- -

*/

OPENROWSET(BULK .[filename, option])can even query the data in the file directly without

loading it to a table or variable It can also be used as the source of anINSERT/UPDATEoperation The

following example queries the XML file directly:

SELECT

x.value(’@ItemNumber’,’CHAR(4)’) AS ItemNumber,

x.value(’@Quantity’,’INT’) AS Quantity,

x.value(’@Price’,’MONEY’) AS Price

FROM (

SELECT CAST(bulkcolumn AS XML) AS data

FROM OPENROWSET(BULK ‘C:\temp\items.xml’, SINGLE_BLOB)

AS x

) a

CROSS APPLY data.nodes(’/Items/Item’) i(x)

/*

- -

*/

To use theOPENROWSET(BULK )option, the user should haveADMINISTRATOR BULK

OPERATIONSpermission

Limitations of the XML data type

Though the XML data type comes with a number of very interesting capabilities, it has a number of

limitations as well However, the limitations are not really ‘‘limiting,’’ considering the extensive set

of functionalities provided by the data type

The stored representation of an XML data type instance cannot exceed 2 GB The term ‘‘stored

represen-tation’’ is important in the preceding statement, because SQL Server converts XML data type values to an

internal structure and stores it This internal representation takes much less space than the textual

rep-resentation of the XML value The following example demonstrates the reduction in size when a value is

stored as an XML data type value:

DECLARE @EmployeeXML XML, @EmployeeText NVARCHAR(500)

SELECT @EmployeeText = ‘

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<EmployeeName>Jacob</EmployeeName>

<EmployeeName>Steve</EmployeeName>

</EmployeeInfo>’

SELECT DATALENGTH(@EmployeeText) AS StringSize /*

StringSize -284

*/

SELECT @EmployeeXML = @EmployeeText SELECT DATALENGTH(@EmployeeXML) AS XMLSize /*

XMLSize -109

*/

The stored representation of the XML data type value is different and much more optimized than the

textual representation and the limit of 2 GB is on the stored representation It indicates that an XML

data type column may be able to store XML documents containing more than 2 * 1024 * 1024 * 1024

VARCHARcharacters

Unlike other data types, XML data type values cannot be sorted or used in a group by expression They

cannot be used in a comparison operation However, they can be used with theIS NULLoperator to

determine if the value isNULL XML data type columns cannot be used in the key of an index They

can only be used in theINCLUDEDcolumn of an index

To facilitate faster querying and searching over XML columns, SQL Server supports a spe-cial type of index called an XML index XML indexes are different from regular indexes and are discussed later in this chapter.

Understanding XML Data Type Methods

The XML data type supports a number of methods that allow various operations on the XML document

The most common operations needed on an XML document might be reading values from elements

or attributes, querying for specific information, or modifying the document by inserting, updating, or

deleting XML elements or attributes The XML data type comes with a number of methods to support all

these operations

Any operation on an XML document is applied on one or more elements or attributes at a

spe-cific location To perform an operation, the location of the spespe-cific element or attribute has to be

specified

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XPath is used for locating XML elements and attributes within an XML document and navigating

through the XML tree Every element and attribute within an XML document has a unique ‘‘path.’’ For

example:

’<Items>

</Items>’

In the preceding example, the path to the firstItemNumberelement is/Items/ItemNumber[1]and

the second is/Items/ItemNumber[2] Each element and attribute within an XML document can be

uniquely identified and processed using an XPath expression

All the XML data type methods accept XPath expressions to specify the target element or attribute on

which the given operation needs to be performed

value()

One of the most useful methods exposed by the XML data type is thevalue()method It is used to

retrieve scalar values from an XML document as a relational column It takes an XQuery expression and

evaluates it to a single node, casts the results to the specified SQL Server data type, and returns the

value Here is an example:

DECLARE @x XML

SELECt @x = ‘<Order OrderID="1" OrderNumber="SO101" />’

SELECT

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

@x.value(’(Order/@OrderNumber)[1]’,’CHAR(5)’) AS OrderNumber

/*

*/

Thevalue()method accepts an XPath expression pointing to the element or attribute to read data

from It also specifies the data type of the result column

nodes()

Thenodes()method returns a row set representation of the XML document An XQuery operation can

be performed on each node returned by the nodes() method This is useful when information has to be

retrieved from all the nodes matching a specific expression Here is an example:

DECLARE @x XML

SELECT @x = ‘

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</Items>’

SELECT x.value(’.’,’CHAR(4)’) AS ItemNumber FROM @x.nodes(’/Items/ItemNumber’) o(x) /*

ItemNumber -1001 1002

*/

The preceding query returns an accessor to all theItemNumberelements in the XML document and

applies thevalue()method on each node

Both of the preceding examples demonstrate how to read information from XML variables There is a

slight difference between the way the query has to be written for XML variables and XML columns

That’s because when working with an XML variable, only one XML document is processed at a time; but

when working with an XML column, several XML documents need to be processed in a single batch

This is usually achieved by using theCROSS APPLYoperator The following example is a modified

version of the preceding query that reads information from an XML column:

SELECT OrderID, x.value(’@ItemNumber’,’CHAR(4)’) AS ItemNumber FROM OrderXML

CROSS APPLY ItemData.nodes(’/Order/Item’) o(x) /*

-

*/

TheCROSS APPLYoperator joins each node returned by thenodes()method with the table, and the

value()method reads theItemNumbervalue from each element returned by thenodes()method

exist()

Theexist()method checks whether an element or attribute specified by a given XPath expression

exists in the document The following query uses theexist()method to filter rows that have a

spe-cific item number:

SELECT OrderID

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FROM OrderXML

WHERE ItemData.exist(’/Order/Item[@ItemNumber = "Z001"]’) = 1

/*

OrderID

-1

*/

Theexist()method returns true (1) if an element or attribute with the specified XPath expression

exists in the XML document

query()

Thequery()method takes an XQuery expression and evaluates it to a list of XML elements that can

be accessed and processed further

This method supports a subset of the XQuery 1.0 specification and allows querying the XML document,

just like a relational database is queried using a SQL query XQuery is very powerful and supports a

number of operators and functions XQuery is discussed later in this chapter under the section

‘‘Under-standing XQuery and FLWOR operations.’’

modify()

Themodify()method is used to perform XML DML operations on an XML document It allows

inserting, updating, or deleting XML elements or attributes within an XML document A detailed

discussion on themodify()method is given later in this chapter in the section ‘‘Performing XML Data

Modification.’’

Joining XML nodes with relational tables

SQL Server has extended the Relational Engine with XQuery capabilities This offers a number of

advantages — for example, the query processor can evaluate relational and XQuery operations in a

single query A single query plan is created with relational and XQuery operations, and results of

a relational query can be joined with XQuery results and vice versa

A T-SQL Developer can take advantage of this and write T-SQL queries that join XML nodes with

rela-tional columns The following code demonstrates a basic example:

SELECT

oh.OrderID,

c.Name AS Customer,

i.ItemDescription AS Item,

x.value(’@Quantity’,’INT’) AS Quantity,

x.value(’@Price’,’MONEY’) AS Price

FROM OrderHeader oh

INNER JOIN OrderXML ox ON

ItemData.value(’(Order/@OrderID)[1]’,’INT’) = oh.OrderID

CROSS APPLY ItemData.nodes(’/Order/Item’) o(x)

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