Tài liệu Using the SQL Server FOR XML Clause ppt

AUTO Specifies that each row in the result set is returned as an XML element The name of the table is used as the name of the tag in the row elements.. Using the RAW Mode You use the RAW

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Using the SQL Server FOR XML Clause

With a standard SQL SELECT statement, you submit your SELECT statement to the database for execution and get results back in the form of rows SQL Server extends the SELECT statement to allow you to query the database and get results back as XML To

do this, you add a FOR XML clause to the end of your SELECT statement The FOR XML clause specifies that SQL Server is to return results as XML

The FOR XML clause has the following syntax:

FOR XML {RAW | AUTO | EXPLICIT}

[, XMLDATA]

[, ELEMENTS]

[, BINARY BASE64]

The RAW, AUTO, and EXPLICIT keywords indicate the XML mode Table 16.1 shows

a description of the keywords used in the FOR XML clause In the next sections, you'll examine some examples of the use of the FOR XML clause

Table 16.1: FOR XML KEYWORDS

KEYWORD DESCRIPTION

RAW Specifies that each row in your result set is returned as an XML <row>

element The column values for each row in the result set become attributes of the <row> element

AUTO Specifies that each row in the result set is returned as an XML element

The name of the table is used as the name of the tag in the row elements EXPLICIT Indicates your SELECT statement specifies a parent-child relationship

This relationship is then used by SQL Server to generate XML with the appropriate nested hierarchy

XMLDATA Specifies that the XML schema is to be included in the returned XML ELEMENTS Specifies that the column values are returned as subelements of the row;

otherwise the columns are returned as attributes of the row You can use this option only with the AUTO mode

BINARY

BASE64

Specifies that any binary data returned by your SELECT statement is encoded in base 64 If you want to retrieve binary data using either the RAW or EXPLICIT mode, then you must use the BINARY BASE64 option

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Using the RAW Mode

You use the RAW mode to specify that each row in the result set returned by your

SELECT statement is returned as an XML <row> element The column values for each row in the result set become attributes of the <row> element

Listing 16.1 shows an example SELECT statement that retrieves the top three rows from the Customers table The results of the SELECT are returned as XML using the FOR XML RAW clause

Listing 16.1: FORXMLRAW.SQL

USE Northwind

SELECT TOP 3 CustomerID, CompanyName, ContactName

FROM Customers

ORDER BY CustomerID

FOR XML RAW

Note This SELECT statement is contained in a T-SQL script named ForXmlRaw.sql, which is located in the sql directory for this chapter

You can load the ForXmlRaw.sql T-SQL script into Query Analyzer by selecting File ➣ Open from the menu You then run the script by selecting Query ➣ Execute, or by

pressing the F5 key Figure 16.1 shows the result of running the script in Query Analyzer You'll notice that the XML is shown on one line, and that the line is truncated

Figure 16.1: Running a SELECT statement containing a FOR XML RAW clause in Query Analyzer

Note By default, the maximum number of characters displayed by Query Analyzer per column is 256 Any results longer than 256 characters will be truncated For the examples in this section, you'll need to increase the maximum number of characters

to 8,192 To do this, you select Tools ➣ Options in Query Analyzer and set the

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Maximum Characters Per Column field to 8,192

Here's the XML line returned by the example, which I copied from Query Analyzer and added some return characters to make it easier to read:

<row

CustomerID="ALFKI"

CompanyName="Alfreds Futterkiste"

ContactName="Maria Anders"/>

<row

CustomerID="ANATR"

CompanyName="Ana Trujillo Emparedados y helados"

ContactName="Ana Trujillo"/>

<row

CustomerID="ANTON"

CompanyName="Antonio Moreno Taquería"

ContactName="Antonio Moreno"/>

Notice that each customer is placed within a <row> tag Also, the column values appear

as attributes within each row; for example, in the first row, the CustomerID attribute is ALFKI

Using the AUTO Mode

You use the AUTO mode to specify that each row in the result set is returned as an XML element The name of the table is used as the name of the tag in the row elements

Listing 16.2 shows an example SELECT statement that retrieves the top three rows from the Customers table The results are returned as XML using the FOR XML AUTO

clause

Listing 16.2: FORXMLAUTO.SQL

USE Northwind

FROM Customers

ORDER BY CustomerID

FOR XML AUTO

The XML returned by this example is as follows:

<Customers

CustomerID="ALFKI"

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ContactName="Maria Anders"/>

<Customers

CustomerID="ANATR"

CompanyName="Ana Trujillo Emparedados y helados"

ContactName="Ana Trujillo"/>

<Customers

CustomerID="ANTON"

CompanyName="Antonio Moreno Taquería"

ContactName="Antonio Moreno"/>

Notice that each customer appears within a <Customer> tag instead of a generic <row> tag, as was the case in the previous RAW mode example

Using the EXPLICIT Mode

You use the EXPLICIT mode to indicate that your SELECT statement specifies a parent-child relationship This relationship is then used by SQL Server to generate XML with the appropriate nested hierarchy

When using the EXPLICIT mode, you must provide at least two SELECT statements The first SELECT specifies the parent row (or rows), and the second specifies the child rows The rows retrieved by the two SELECT statements are related through special columns named Tag and Parent Tag specifies the numeric position of the element, and Parent specifies the Tag number of the parent element (if any)

Let's consider an example that uses two SELECT statements The first SELECT retrieves the CustomerID, CompanyName, and ContactName for the row from the Customers table that has a CustomerID of ALFKI The second SELECT additionally retrieves the

OrderID and OrderDate from the row in the Orders table that also has a CustomerID of ALFKI The first SELECT statement is as follows:

SELECT

1 AS Tag,

0 AS Parent,

CustomerID AS [Customer!1!CustomerID],

CompanyName AS [Customer!1!CompanyName],

ContactName AS [Customer!1!ContactName],

NULL AS [Order!2!OrderID!element],

NULL AS [Order!2!OrderDate!element]

FROM Customers

WHERE CustomerID = 'ALFKI'

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The Tag column specifies the numeric position of the row in the XML hierarchy The Parent column identifies the parent, which is 0 in the previous SELECT statement; that's because this row is the parent, or root, in the XML hierarchy

Note You can also use a Tag value of NULL to indicate the root.

The CustomerID, CompanyName, and ContactName columns in the previous SELECT are supplied an alias using the AS keyword, followed by a string that uses the following format:

[elementName!tag!attributeName!directive]

where

• elementName specifies the name of the row element in the returned XML

• tag specifies the Tag number

• attributeName specifies the name of the column elements in the returned XML

• directive (optional) specifies how the element is to be treated in the XML The directives are shown in Table 16.2

Table 16.2: DIRECTIVES

DIRECTIVE DESCRIPTION

element Indicates that the column value appears as a contained row element

within the outer row element, rather than an embedded attribute of the outer row element The element directive may be combined with ID, IDREF, or IDREFS

hide Indicates that the column value doesn't appear in the returned

XML

xml Similar to the element directive except that the column value isn't

coded as an entity in the returned XML This means that the special characters &, ', >, <, and "are left as is These characters would otherwise be coded as &, ', >, <, and " respectively The xml directive may be combined with hide

xmltext Indicates that the column value is contained in a single tag To use

the xmltext directive, your column type must be varchar, nvarchar, char, nchar, text, or ntext

cdata Indicates that the column value is contained within a CDATA

section CDATA sections are used to escape blocks of text containing special characters that would otherwise be interpreted as markup; these characters include &, ', >, <, and " To use the cdata directive, your column type must be varchar, nvarchar, text, or

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Table 16.2: DIRECTIVES

DIRECTIVE DESCRIPTION

ntext

ID Indicates that the column value is an ID attribute An IDREF and

IDREFS attribute can point to an ID attribute, allowing you to create links within the XML

IDREF Indicates that the column value is an IDREF attribute

IDREFS Indicates that the column value is an IDREFS attribute

Let's consider an example: CustomerID AS [Customer!1!CustomerID] specifies that the CustomerID column value will appear within the Customer row element with the

attribute name of CustomerID

After the ContactName in the previous SELECT clause, appear two NULL columns; these are used as placeholders for the OrderID and OrderDate columns that are retrieved

by the second SELECT statement, which you'll see next These two columns use the element directive, which indicates that the column values are to appear as contained elements within the Customer row element

The second SELECT statement retrieves the rows from the Orders table that has a CustomerID of ALFKI:

SELECT

2 AS Tag,

1 AS Parent,

C.CustomerID,

C.CompanyName,

C.ContactName,

O.OrderID,

O.OrderDate

FROM Customers C, Orders O

WHERE C.CustomerID = O.CustomerID

AND C.CustomerID = 'ALFKI'

Notice that the Parent column is set to 1, which indicates that the parent is the row previously retrieved by the first SELECT statement shown earlier

Listing 16.3 shows a complete example that uses the two SELECT statements shown in this section

Listing 16.3: FORXMLEXPLICIT.SQL

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USE Northwind

SELECT

1 AS Tag,

0 AS Parent,

CustomerID AS [Customer!1!CustomerID],

CompanyName AS [Customer!1!CompanyName],

ContactName AS [Customer!1!ContactName],

NULL AS [Order!2!OrderID!element],

NULL AS [Order!2!OrderDate!element]

FROM Customers

WHERE CustomerID = 'ALFKI'

UNION ALL

SELECT

2 AS Tag,

1 AS Parent,

C.CustomerID,

C.CompanyName,

C.ContactName,

O.OrderID,

O.OrderDate

FROM Customers C, Orders O

WHERE C.CustomerID = O.CustomerID

AND C.CustomerID = 'ALFKI'

FOR XML EXPLICIT

Note The UNION ALL clause causes the results retrieved by the two SELECT statements

to be merged into one result set

The combined result set produced by the UNION ALL clause is then converted to XML

by the FOR XML EXPLICIT clause

The XML returned by the example is as follows:

<Customer

CustomerID="ALFKI"

ContactName="Maria Anders">

<Order>

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</Order>

<Order>

</Order>

<Order>

</Order>

<Order>

</Order>

<Order>

</Order>

<Order>

</Order>

</Customer>

Notice that the OrderID and OrderDate elements appear as row elements contained in the outer Order element That's because the element directive was specified for the OrderID and OrderDate elements in the first SELECT statement If the element directive is

omitted from the OrderID and OrderDate elements, then the returned XML is as follows:

<Customer

CustomerID="ALFKI"

ContactName="Maria Anders">

</Customer>

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Notice that the OrderID and OrderDate elements are embedded as attributes of the outer Order element

Using the XMLDATA Option

You use the XMLDATA option to specify that the XML schema document type

definition (DTD) is to be included in the returned XML The XML schema contains the name and type of the column attributes

Listing 16.4 shows an example that uses the XMLDATA option to return the XML schema along with the ProductID, ProductName, and UnitPrice columns for the top two rows from the Products table

Listing 16.4: FORXMLAUTOXMLDATA.SQL

USE Northwind

SELECT TOP 2 ProductID, ProductName, UnitPrice

FROM Products

ORDER BY ProductID

FOR XML AUTO, XMLDATA

Note In this example, I use columns from the Products table rather than the Customers table because the Customers table contains only string column values, and I want you to see some of the different types returned in an XML schema The Products

table contains column values that consist of both strings and numbers

The ProductID column is of the SQL Server int type, ProductName is of the nvarchar type, and UnitPrice is of the money type The XML returned by this example is as follows:

<Schema name="Schema3"

xmlns="urn:schemas-microsoft-com:xml-data"

xmlns:dt="urn:schemas-microsoft-com:datatypes">

</ElementType>

</Schema>

<Products

xmlns="x-schema:#Schema3"

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ProductID="1"

ProductName="Chai"

UnitPrice="18.0000"/>

<Products

xmlns="x-schema:#Schema3"

ProductID="2"

ProductName="Chang"

UnitPrice="19.0000"/>

Notice the different XML types of the ProductID, ProductName, and UnitPrice attributes specified in the AttributeType tag near the start of the previous XML

For more information, see XML Schemas by Chelsea Valentine, Lucinda Dykes, and Ed

Tittel (Sybex, 2002)

Using the ELEMENTS Option

You use the ELEMENTS option to indicate that the column values are returned as

subelements of the row; otherwise the column values are returned as attributes of the row

Tip You can use the ELEMENTS option only with the AUTO mode.

Listing 16.5 shows an example that uses the ELEMENTS option when retrieving the top two rows from the Customers table

Listing 16.5: FORXMLAUTOELEMENTS.SQL

USE Northwind

FROM Customers

ORDER BY CustomerID

FOR XML AUTO, ELEMENTS

The XML returned by this example is as follows:

<CustomerID>ALFKI</CustomerID>

<CompanyName>Alfreds Futterkiste</CompanyName>

<ContactName>Maria Anders</ContactName>

</Customers>

<CustomerID>ANATR</CustomerID>

<CompanyName>Ana Trujillo Emparedados y helados</CompanyName>

Tiêu đề	Using the SQL Server For XML Clause
Trường học	University of Information Technology
Chuyên ngành	Computer Science
Thể loại	Tài liệu
Thành phố	Ho Chi Minh City

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