Hướng dẫn học Microsoft SQL Server 2008 part 52 ppsx

Nielsen c18.tex V4 - 07/21/2009 1:01pm Page 472 Part III Beyond Relational <Orders xmlns="http://www.sqlserverbible.com/orders"> <Order CustomerID="1">SO101</Order> <Order CustomerID="1">SO102</Order> </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 XMLNAMESPACES offers 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.’’ The query() method of the XML data 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 the query() method is to retrieve one or more XML nodes from the given XML document. The result of the query() method is always an XML data type value: SELECT ItemData.query(’/Order/Item’) FROM OrderXML 472 www.getcoolebook.com Nielsen c18.tex V4 - 07/21/2009 1:01pm Page 473 Manipulating XML Data 18 /* <Item ItemNumber="D001" Quantity="1" Price="900" /> <Item ItemNumber="Z001" Quantity="1" Price="200" /> <Item ItemNumber="D001" Quantity="1" Price="900" /> */ The query() 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 /* <Item ItemNumber="D001" Quantity="1" Price="900" /> <Item ItemNumber="D001" Quantity="1" Price="900" /> */ The result of the query() 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 12 21 */ This example used the XQuery count() method to retrieve the number of Item elements in each row. The query() method always returns an XML data type value; hence, the value() method is used to retrieve an INT value from it. FLWOR operation ThetruepowerofXQuerycomeswiththeFLWOR operation, which is pronounced like ‘‘flower’’ and stands for FOR LET WHERE ORDER BY and RETURN.AFLWOR operation 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. Abasic FLWOR query using FOR and RETURN is shown in the following example: DECLARE @x XML SELECT @x = ‘ <Items> <ItemNumber>1003</ItemNumber> 473 www.getcoolebook.com Nielsen c18.tex V4 - 07/21/2009 1:01pm Page 474 Part III Beyond Relational <ItemNumber>1004</ItemNumber> </Items>’ SELECT @x.query(’ for $item in Items/ItemNumber return $item ‘) /* <ItemNumber>1003</ItemNumber> <ItemNumber>1004</ItemNumber> */ WHERE and ORDER BY can be specified to filter and sort the output: DECLARE @x XML SELECT @x = ‘ <Items> <ItemNumber>1003</ItemNumber> <ItemNumber>1004</ItemNumber> <ItemNumber>1001</ItemNumber> <ItemNumber>2007</ItemNumber> <ItemNumber>3009</ItemNumber> <ItemNumber>4005</ItemNumber> </Items>’ SELECT @x.query(’ for $item in Items/ItemNumber where $item[. < "2000"] order by $item return $item ‘) /* <ItemNumber>1001</ItemNumber> <ItemNumber>1003</ItemNumber> <ItemNumber>1004</ItemNumber> */ The WHERE condition in the preceding XQuery expression filters the nodes for ItemNumber less than 2000. The ORDER BY clause then orders the nodes by ItemNumber. A FLWOR operation can be used to completely restructure an XML document, as demonstrated here: DECLARE @x XML SELECT @x = ‘ <Item ItemNumber="D001" Quantity="1" Price="900" /> <Item ItemNumber="Z001" Quantity="1" Price="200" />’ 474 www.getcoolebook.com Nielsen c18.tex V4 - 07/21/2009 1:01pm Page 475 Manipulating XML Data 18 SELECT @x.query(’ for $item in Item return <ItemNumber> {data($item/@ItemNumber)} </ItemNumber> ‘) /* <ItemNumber>D001</ItemNumber> <ItemNumber>Z001</ItemNumber> */ The preceding example transforms the ItemNumber attributes to elements and produces a completely different XML document. Complex FLWOR operations 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 the let clause in FLWOR operations. The let clause allows declaring and using inline variables within the XQuery expression used in a FLWOR query: DECLARE @x XML SELECT @x = ‘ <Item ItemNumber="D001" Quantity="2" Price="900" /> <Item ItemNumber="Z001" Quantity="3" Price="200" />’ SELECT @x.query(’ for $item in Item let $itm := $item/@ItemNumber let $tot := $item/@Quantity * $item/@Price return <Item> <ItemNumber>{data($itm)}</ItemNumber> <TotalPrice>{data($tot)}</TotalPrice> </Item> ‘) /* <Item> <ItemNumber>D001</ItemNumber> <TotalPrice>1800</TotalPrice> </Item> <Item> <ItemNumber>Z001</ItemNumber> <TotalPrice>600</TotalPrice> </Item> */ 475 www.getcoolebook.com Nielsen c18.tex V4 - 07/21/2009 1:01pm Page 476 Part III Beyond Relational The XQuery expression in the preceding example declares two inline variables using the let clause: $itm and $tot. These variables are initialized within the XQuery expression and used to generate cus- tom XML nodes. The let clause reduces the complexity of FLWOR operations 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() The string() function is a data accessor function that returns the string value from an element or attribute. The concat() function joins two string values, and the substring() 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, the contains() 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() 476 www.getcoolebook.com Nielsen c18.tex V4 - 07/21/2009 1:01pm Page 477 Manipulating XML Data 18 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-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 worthamentionhere: ■ 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() and position() ■ QName functions: expanded-QName(), local-name-from-QName() and namespace- uri-from-QName ■ SQL Server XQuery extension functions: sql:variable() and sql: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 The modify() method of the XML data 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. The insert, replace value of,anddelete instructions are used with the modify() method to perform INSERT, UPDATE,andDELETE operations on XML documents. Each of these operations has its own syntax and is briefly explained in the following sections. 477 www.getcoolebook.com Nielsen c18.tex V4 - 07/21/2009 1:01pm Page 478 Part III Beyond Relational Insert operation A new element or attribute can be inserted into an XML document by using the modify() method with the insert command. A basic example demonstrating an insert operation 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 OrderNumber="SO101"> <CustomerID>1</CustomerID> </SalesOrder> */ Update operation The modify() method can be used with replace value of command to modify the value of ele- ments or attributes. The following example changes the value of the CustomerID element: DECLARE @x XML SELECT @x = ‘ <SalesOrder OrderNumber="SO101"> <CustomerID>1</CustomerID> </SalesOrder>’ DECLARE @CustomerID INT SELECT @CustomerID = 2 SET @x.modify(’ replace value of (SalesOrder/CustomerID/text())[1] with sql:variable("@CustomerID") ‘) SELECT @x /* <SalesOrder OrderNumber="SO101"> <CustomerID>1</CustomerID> </SalesOrder> */ 478 www.getcoolebook.com Nielsen c18.tex V4 - 07/21/2009 1:01pm Page 479 Manipulating XML Data 18 Delete operation The modify() method can be used with the delete instruction to remove an element or attribute from a given XML document. The following example deletes the CustomerID element from the XML document: DECLARE @x XML SELECT @x = ‘ <SalesOrder OrderNumber="SO101"> <CustomerID>1</CustomerID> </SalesOrder>’ SET @x.modify(’ delete (SalesOrder/CustomerID)[1] ‘) SELECT @x /* <SalesOrder OrderNumber="SO101" /> */ What’s new for XML DML operations in SQL Server 2008 SQL Server 2005’s implementation of the modify() method does not support XML variables with the insert instruction. 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 to VARCHAR/NVARCHAR and per- form an insert operation. However, in such a case, SQL Server will encode the XML tags within the VARCHAR/NVARCHAR value, which may not be the desired result most of the time. SQL Server 2008 enhanced the modify() method to support XML variables with the insert com- mand. The following code snippet shows an example: DECLARE @doc XML, @val XML SELECT @doc = ‘ <SalesOrder OrderNumber="SO101"> <CustomerID>1</CustomerID> </SalesOrder>’ SELECT @val = ‘ <Items> <Item ItemNumber="Z001" Quantity="1" Price="900"/> </Items>’ SET @doc.modify(’ insert sql:variable("@val") as last into (SalesOrder)[1] ‘) SELECT @doc 479 www.getcoolebook.com Nielsen c18.tex V4 - 07/21/2009 1:01pm Page 480 Part III Beyond Relational /* <SalesOrder OrderNumber="SO101"> <CustomerID>1</CustomerID> <Items> <Item ItemNumber="Z001" Quantity="1" Price="900" /> </Items> </SalesOrder> */ Note that the syntax of the insert command 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. The WITH NAMESPACES directive can be used to declare the XML namespaces and refer to them in the XQuery expressions following the declaration: DECLARE @x XML SELECT @x = ‘ <SalesOrder xmlns="http://www.sqlserverbible.com/order" xmlns:cust="http://www.sqlserverbible.com/customer"> <OrderID>1</OrderID> <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 1 10001 */ The WITH XMLNAMESPACES directive simplifies namespace declaration, as the namespaces can be declared once and then reused in the XQuery expression that follows the declaration. 480 www.getcoolebook.com Nielsen c18.tex V4 - 07/21/2009 1:01pm Page 481 Manipulating XML Data 18 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 basic OPENXML() function call: DECLARE @hdoc INT DECLARE @xml VARCHAR(MAX) SET @xml =’ <SalesOrder OrderNumber="SO101"> <Items> <Item ItemNumber="D001" Quantity="1" Price="900.0000" /> <Item ItemNumber="Z001" Quantity="1" Price="200.0000" /> </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 SO101 D001 1 900.00 SO101 Z001 1 200.00 */ As is apparent from the preceding e xample, a call to OPENXML() 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 the OPENXML() function call as an a rgument. Finally, the handle has to be released to free system resources used for the operation. 481 www.getcoolebook.com . 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. for XML DML operations in SQL Server 2008 SQL Server 2005’s implementation of the modify() method does not support XML variables with the insert instruction. With SQL Server 2005, it is not possible. local-name-from-QName() and namespace- uri-from-QName ■ SQL Server XQuery extension functions: sql: variable() and sql: column() The XQuery implementation of SQL Server 2008 still does not support user-defined