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As you ve seen, the FOR XML clause of SQL Server allows you to retrieve relational data in XML format. However, there is another way to do it the OPENXML function, which allows you to read XML data in a relational fashion. Suppose that you have XML markup that contains a list of employees, and your aim is to import this list into your Employees table. In the absence of something like OPENXML, accomplishing this task would be tedious. As you will soon see, the OPENXML function makes your job much easier. Listing 10-15 shows the source XML markup containing the employee listing. Listing 10-15. The Source XML Markup <Employees> <Employee EmployeeID="10" FirstName="John" LastName="Moore" /> <Employee EmployeeID="11" FirstName="Bill" LastName="Short" /> </Employees> As you can see, the root element of the markup is <Employees>. Further, it contains <Employee> elements representing employee records. The EmployeeID, FirstName, and LastName appear as attributes of the <Employee> element. To read any XML markup by using the OPENXML function, you need to perform the following steps:
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CHAPTER 10 XML IN S QL SERVE R
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1. Prepare and load the XML document for processing. 2. Call the OPENXML function as per your need. 3. Remove the loaded XML document from memory. These three steps are illustrated in Listing 10-16. Listing 10-16. Using the OPENXML Function SET IDENTITY_INSERT Employees ON DECLARE @hDoc INT DECLARE @xml VARCHAR(1000) SET @xml= '<Employees> <Employee EmployeeID="10" FirstName="John" LastName="Gates" /> <Employee EmployeeID="11" FirstName="Bill" LastName="Short" /> </Employees>' EXEC sp_xml_preparedocument @hDoc OUTPUT, @xml INSERT INTO EMPLOYEES (EMPLOYEEID,FIRSTNAME,LASTNAME) ( SELECT * FROM OPENXML(@hDoc,'Employees/Employee',0) WITH (EmployeeID int,FirstName varchar(50),LastName varchar(50)) ) EXEC sp_xml_removedocument @hDoc The script in Listing 10-16 declares two variables named hDoc and xml. The integer variable hDoc is used later for storing a handle to the loaded XML document. The VARCHAR variable xml is used to store the XML markup shown in Listing 10-15 as a string. The SET statement assigns the XML markup to the xml variable. Then we call the sp_xml_preparedocument system stored procedure, which parses and loads the supplied XML markup in memory. It returns a handle to the loaded document in the form of an integer. Next, this handle is collected in the hDoc variable that we declared earlier. Then an INSERT statement is executed, making use of the OPENXML function. Observe the call to OPENXML carefully. The OPENXML function is used in a SELECT statement as if it were a table. It accepts three parameters: The first parameter is a handle to the XML data loaded by using sp_xml_preparedocument. The second parameter is an XPath pattern pointing to the node of the XML data that is to be treated as a row. In our example, this base path is Employees/Employee. The third parameter is a flag indicating the mapping between the XML data and the relational rowset. The third parameter can take values as shown in Table 10-2.
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CHAPTER 10 XML IN S QL SERVE R
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Table 10-2. Mapping Between XML Data and a Relational Rowset
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Flag Value
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Specifies that attributes of the XML elements are supplying column values for the relational rowset. This is the default. Specifies that attributes of the XML elements are supplying column values for the relational rowset. When combined with a flag value of 2, attributes are picked up as column values and then element values are assigned to the remaining columns. Specifies that elements of the source XML are supplying column values for the relational rowset. This flag can be combined with 1 or 2 and indicates that the consumed data should not be copied to the overflow property @mp:xmltext.
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Further, the WITH clause of OPENXML specifies the structure of the resultant rowset. The structure can be specified as a comma-separated list of column names and their data types. In our example, we have three columns: EmployeeID, FirstName, and LastName. Note that these column names are the same as the attribute names in the source XML markup. Thus the rowset returned from the SELECT statement and OPENXML is fed to the INSERT statement. The INSERT statement then adds the data to the Employees table. In our example, it will add two rows. After the INSERT operation is done, the XML document is removed from memory by using another system stored procedure: sp_xml_removedocument. This accepts the handle of an XML document loaded previously by using sp_xml_preparedocument and cleans up the memory consumed by the document. Calling sp_xml_removedocument is very important because failing to do so can waste valuable memory of your application.
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Note A thorough discussion of the OPENXML clause is outside the scope of this book. The aim here is to
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