Types of Secondary XML Indexes in SQL Server

These are non-clustered index of the primary XML index. There must be a primary xml index before each secondary xml index. Following are some types of secondary xml indexes:

Path Indexes

The path index is built on the path value columns of the primary XML indexes. This index improves the performance of queries that use paths and values to select data.

For example, if you execute a query that checks for the existence of a product model ID using an XQuery expression as /PD:ProductDescription/@ProductModelID[.=”19”], you can create a path secondary index on the CatalogDescription column of the ProductModel table. In this path index, you can use the primary index created previously.

The following statement creates a Path index on the CatalogDescription column:

CREATE XML INDEX PIdx_ProductModel_CatalogDescription_PATH ON Production.ProductModel (CatalogDesctiption)USING XML INDEX PXML_ProductModel_CatalogDescription FOR PATH

The preceding code create a path index, Pldx_ProductModel_CatalogDesctiption_PATH

Value Indexes

The value indexes contain the same items as path indexes but in the reverse order. It contains the value of the column first and then the path id. This index improves the performance of queries that use paths to select data.

For example, if you execute a query that checks the existence of a node in an XQuery expression such as//Item@ProductID=”1”], you can create a value secondary index by using the primary index created previously.

The following statement creates a value index on the CatalogDesctiption column:

CREATE XML INDEX PIdx_ProductModel_CatalogDesctiption_VALUE ON Production.ProductModel (CatalogDesctiption)
USING XML INDEX PXML_ProductModel_CatalogDescription
FOR VALUE

The preceding code creates a value index,
PIdx_ProductModel_CatalogDescription_VALUE on the CatalogDescription column of the table.

Property Indexes

The property index contains the primary key of the base table, path id, and the clause columns of primary XML indexes. This index improves the performance of queries that use paths to select data.

For example, if you execute a query that returns a value of the node in an XQuery expression, such as /ItemList/Item/@ProductID)[1], you can create a property secondary index on the CatalogDescription column of the ProductModel table by using the following statement:

CREATE XML INDEX PIdx_ProductModel_CatalogDescription_PROPERTY ON Production.ProductModel (CatalogDescription)
USING XML INDEX PXML_ProductModel_CatalogDescription FOR PROPERTY

The preceding code creates a property index, PIdx_ProductModel_CatalogDescription_PROPERTY, on the CatalogDescription column of the table.

You need to consider the following guidelines while creating an XML index:

• XML indexes can be created only on XML columns.
• XML indexes only support indexing a single XML column.
• XML indexes can only be added to tables, views, table-valued variables with XML column or XML variables.
• XML indexes created on a table do not allow you to modify the primary key. To do so, you first need to drop all the XML indexes on the table.

Types of XML Indexes used in SQL Server: Speed up Execution

When a query is based on an XML column, the query processor needs to parse the XML data each time the query is executed. In SQL Server, and XML data value can be of a maximum of two gigabytes (GB).

Therefore, the XML values can be very large and the server might take time to generate the result set. To speed up the execution of the query based on the XML data type, SQL Server allows you to create an index that is based on columns storing XML data values. Such indexes are called XML indexes.

Primary XML Index

This is a clustered B-Tree representation of the nodes in the XML data. When an index is created on a column with the XML data type, an entry will be created for all the nodes in the XML data. Therefore, the index creates several rows of data for each XML value in the column.

You can create XML indexes on XML columns by using the CREATE PRIMARY XML INDEX and CREATE XML INDEX T-SQL commands. For example, the ProductModel table contains the CatalogDescription column that stores XML values. You can create a primary XML index on this column by using the following statement:

CREATE PRIMARY XML INDEX PXML_ProductModel_CatalogDesctiption ON Production.ProductModel (CatalogDescription)

The preceding statement will create an index for all the nodes in the XML data stored in the CatalogDescription column.

Secondary XML Index

This is a non-clustered index of the primary XML index. A primary XML index must exist before any secondary index can be created. After you have created the primary XML index, an additional three kinds of secondary XML indexes can be defined on the table. The secondary XML indexes assist in the XQuery processing.

The three types of secondary XML indexes are:

• Path indexes
• Value indexes
• Property indexes

How to Modify XML Data using Functions in SQL Server

Similar to any other type of data, programmer might also need to modify the XML data. To modify data, you can use the modify function provided by the XML data type of the SQL Server. The modify function specifies an XQuery expression and a statement that specifies the kind of modification that needs to be done.

This function allows you to perform the following modifications:

• Insert: Used to add nodes to XML in an XML column or variable. For example, the management of AdventureWorks wants to add another column specifying the type of customer, in the CustDetails table. The default value in the Type column should be ‘Credit’. To resolve this problem, the database developer of AdventureWorks will create the following query:
  
  UPDATE CusomtDetails SET Cust_Details.modify (‘ inser attribute Type{“Credit”} as first into (/Customer) [1]’)
  
• Replace: Used to update the XML data. For example, James Stephen, one of the customers of AdventureWorks, has decided to change his customer type from Credit to Cash. As a database developer, you can create the following query to reflect this change:
  
• Delete: Used to remove a node from the XML data. For example, the management of AdventureWorks has decided to remove the ‘City’ column from the customer details. You can write the following query to display the results:
  
  UPDATE CustomDetails SET Cust_Details.modify (‘delete (/Customer/@City) [1]’)

Retreive XML data using XQuery

How to Retrieve XML Data Using XQuery

In addition to FOR XML, SQL Server allows programmer to extract data stored in variables or columns with the XML data type by using XQuery. XQuery is a language that uses a set of statements and functions provided by the XML data type to extract data. As compared to the FOR XML clause of the SELECT statement, the XQuery statements allow you to extract specific parts of the XML data.

Each XQuery statement consists of two parts, prolog and body. In the prolog section, you declare the namespaces. In addition, schemas can be imported in the prolog. The body parts specifies the XML nodes to be retrieved. The XQuery language includes the following statements:

• For: Used to iterate through a set of nodes at the same level as in an XML document.
• Let: Used to declare variables and assign values.
• Order by: Used to specify a sequence.
• Where: Used to specify criteria for the data to be extracted.
• Return: Used to specify the XML returned from a statement.

The XQuery statements also use the following functions provided by the XML data type:

Query: Used to extract XML from an XML data type. The XML to be retrieved is bicycle is manufactured at AdventureWorks, it passes through a series of work centre locations. Each work centre location produces a different cycle component. Therefore, the number of production steps varies between different work centres.

To analyse the production process, the management of AdventureWorks needs to retrieve a list of the location IDs of all the work centers, which have more than four steps. You need to generate the list displaying the location ids in the ascending order of the steps included in the work centres.

To perform this task, the database developer can create the following query:

SELECT Instructons.query
(‘ declare namespace
ns=http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelManuInstructions:
for $work in /ns:root/ns:Location
where count(#work/ns:step) > 4
order by count ($work/ns:step)
return
count($work/ns:step)’) AS Result
FROM Production.ProductModel
WHERE Instructions IS NOT NULL

Value: Used to return a single value from an XML document. To extract a single value, you need to specify an XQuery expression that identifies a single node and a data type of the value to be retrieved.

For example, the management of AventureWorks, Ins. Wants a list containing the product model id, product name, machine hours, and labour hours. However, not all product have production instructions. As a database developer, you have stored this data in the XML format in the ProductModel table. You can create the following query to display the results:

SELECT ProductModelID, Name, Instructions.value (‘declare namespace ns=”http//schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductManuInstructions”;
(/ns:root/ns:Location/@LaborHours) [1]’, ‘float’)AS
LaborHours,
Instructions.value(declare namespace
Ns=”http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelManuInstructions”;
(/ns:root/ns:Location/@MachineHours) [1]’, ‘float’) AS MachineHours
FROM Production.ProductModel
WHERE Instructions IS NOT NULL

Exist: Used to check the existence of a node in an XML data. The function returns I if the specified node exists else it returns 0. For example, the management of AdventureWorks, wants the details of all the customers in the city ‘NJ’. The details of all the customers are stored in an XML format in the CustDetails table. You can use the following query to display the results:

SELECT Cust_ID, Cust_Details.exist
(‘Customer[@City=’NJ”]’) AS ‘True’ FROM CustDetails

Retrieve XML data from DataSet

How to Retrieve XML Data from a Database Table: SQL

At times, you need to retrieve the relational data from a table into the XML format for reporting purposes or to share the data across different applications. This involves extracting data from a table in the form of well-formed XML fragments. You can retrieve the XML data in the following ways:

• Using the FOR XML clause in the SELECT statement
• Using XQuery

Using the FOR XML Clause in the SELECT statement

SQL Server allows you to extract data from relational tables into an XML format by using the SELECT statement with the FOR XML clause. You can use the FOR XML clause to retrieve the XML data by using the following modes:

• RAW
• AUTO
• PATH
• EXPLICIT

Using the RAW Mode

The RAW mode is used to return an XML file with each row representing an XML element. The RAW mode transforms each row in the query result set into an XML element with the element name row. Each column value that is not NULL is mapped to an attribute with the same name as the column name.

The following statements display the details of employees with employee ID as 1 or 2:
SELECT EmployeeID, ContactID, LoginID, Title
FROM HumaneResources.Employee
WHERE EmployeeID=1 OR EmployeeID=2
FOR XML RAW

The preceding query displays the employee details in the following format:

<row EmployeeID=”1” ContactID=”1209” LoginID=”adventure-works/guy1” Title=”Production Technician – WC60” />
<row EmployeeID=”2” ContactID=”1030” LoginID=”adventure-works/kevin0” Title=”Marketing Assistant” />

Using the AUTO Mode

The AUTO mode is used to return query results as nested XML elements. Similar to the RAW mode, each column value that is not NULL is mapped to an attribute that is named after either the column name or the column alias. The element that these attributes belong to is named to the table that they belong to or the table alias that is used in the SELECT statement, as shown in the following query:

SELECT EmployeeID, ContactID, LoginID, Title
FROM HumanResources.Employee Employee
WHERE EmployeeID=1 OR EmployeeID=2
FOR XML AUTO

If the optional ELEMENTS directive is specified in the FOR XML clause, the columns listed in the SELECT clause are mapped to sub-elements, as shown in the following query:

SELECT EmployeeID, ContactID, LoginID, Title
FROM HumanResources.Employee Employee
WHERE EmployeeID=1 OR EmployeeID=2
FOR XML AUTO, ELEMENTS

Using the PATH Mode

The PATH mode is used to return specific values by indicating the column names for which you need to retrieve the data, as shown in the following query:

SELECT EmployeeID “@EmpID”,
FirstName “EmpName/First”,
MiddleName “EmpName/Middle”,
LastName “EmpName/Last”
FROM HumanResources.Employee e JOIN Person.Contact c
AND e.EmployeeID=1
FOR XML PATH

The preceding query displays the output in the following format:
<row EmpID=”1”>
<EmpName>
<First>Guy</First>
<Middle>R</Middle>
<Last>Gilbert</Last>
</Employee>
</row>

In the preceding result set, the EmployeeID column is mapped to the EmpID attribute with @ sign. The FirstName, MiddleName, and LastName columns are mapped as subelements of the EmpName element with the slash mark(/)

You can also use the optional ElementName argument with the PATH mode query to modify the name of the default row element, as shown in the following query:

SELECT EmployeeID “@EmpID”,
FirstName “EmpName/First”,
MiddleName “EmpName/Middle”,
LastName “EmpName/Last”
FROM HumanResources.Employee e JOIN Person.Contact c ON e.ContactID = c.ContactID
AND e.EmployeeID=1
FOR XML PATH (‘Employee’)

Using the EXPLICIT Mode

The EXPLICIT mode is used to return an XML file that obtains the format as specified in the SELECT statement. Separate SELECT statement can be combined with the UNION ALL statement to generate each level/element in the resulting XML output. Each of these SELECT statements requires the first two tags to be called Tag and Parent. The Parent element is used to control the nesting of elements. It contains the tag number of the parent element of the current element. The top-level element in the document should have the Parent value set to 0 or NULL.

For example, the managers of AdventureWorks want to access the information regarding products through their mobile devices. These devices cannot directly connect to the SQL Server, but can read the data provided in the XML format. Therefore, you need to convert the details of the products from the Product table into the XML document. To perform this task, you need to create an XML document with <Product> as the parent tag. The <Product> tag will contain ProductID as an attribute and <ProductName> and <Color> as child elements.

To perform this task, the database developer can create the following query:

SELECT 1 AS Tag,
NULL AS Parent,
ProductID AS [Product!1!ProductID],
Name AS [Product!1!ProductName!element],
Color AS [Product!1!Color!elementxsinil]
FROM Production.Product
FOR XML EXPLICIT

How to Store Typed XML Data in XML Columns: SQL

To store the typed XML data, programmer need to first register the schema associated with the data in the XML schema collection objects in the database. The XML schema collection is an object on the SQL Server that is used to save one or more XML schemas. You can create an XML schema collection object by using the following SQL statement:

CREATE XML SCHEMA COLLECTION <Name> as Expression
Where,

• Name specifies an identifier name with which the SQL Server will identify the schema collection.
• Expression specifies an XML value that contains one or more XML schema documents

For example, the customer details are associated with the following schema:
<?XML version=”1.0” ?>
<xsd:schema targetNamespace=”http//schemas.adventure-
Works.com/Customers” xmlns=”http//shemas.adventure-
Works.com/Customers” elementFormDefault=”qualified”
attributeFormDefault=” unqualified”
xmlns:xsd=”http://www.w3.org/2001/XMLSchema”?>
<xsd:element name =”Customer” type=”xsd:string” />
<xsd:attribute name=”City” type=”xsd:string” />
</xsd:schema>

You can use the following statements to register the preceding schema, named as CustomerSchemaCollection, with the database:

CREATE XML SCHEMA COLLECTION CustomerSchemaCollection AS n’<?xml version= “1.0” ?>
<xsd:schema targetNamespace=”http://schemas.adventure-
Works.com/Customers” xmlns=”http://schemas.adventure-
Works.com/Customers” elementFormDefault=”qualified”
attributeFormDefault=”unqualified”
xmlns:xsd=”http://www.w3.org/2001/XMLSchema”.>
<xsd:element name =”Customer” type=”xsd:string” />
<xsd:attribute name=”Name” type=”xsd:string” />
<xsd:attribute name=”City” type=”xsd:string” />
</xsd:schema>’

You can view information about the registered schemas in a database by querying the sys.XML_schema_collection catalog view, as shown in the following statement:
SELECT * FROM sys.XML_schema_collections

After registering the XML schema, you can use the schemas to validate typed XML values while inserting records into the tables. You need to specify this while creating a table that will store the XML data. In the preceding example, if you need to validate the customer details with the CustomerSchemaCollection schema, you need to create the CustDetails table by using the following statement:

CREATE TABLE CustDetails
(
CustID int,
CustDetails XML
)

You can insert data into this table by using the following statement:
INSERT INTO CustDetails VALUES (2, ‘<?xml version=”1.0”?> <CustomerName=”Abrahim Jones” City=”Selina” />’)

While executing the preceding statement, the SQL Server will validate the values for the CustDetails column against the CustomerSchemaCollection schema.