In SQL Server, the XML data type provides several methods that allow you to query, modify, and manipulate XML data effectively. Two of the most commonly used methods for querying XML data are query() and value(). Here's how you can use them:
XML Data Type Methods
1. query():
- The query() method is used to return an XML fragment from the XML instance.
- It accepts an XQuery expression and returns an XML value.
Example:
DECLARE @xmlData XML =
'<root>
<person>
<name>John</name>
<age>30</age>
</person>
<person>
<name>Jane</name>
<age>25</age>
</person>
</root>'
SELECT @xmlData.query('/root/person')
This query will return the <person> elements as an XML fragment.
2. value():
- The value() method is used to extract a scalar value from the XML instance.
- It also takes an XQuery expression as input but requires a type specification for the return value.
Example:
DECLARE @xmlData XML =
'<root>
<person>
<name>John</name>
<age>30</age>
</person>
<person>
<name>Jane</name>
<age>25</age>
</person>
</root>'
SELECT @xmlData.value('(/root/person)[1]/name[1]', 'nvarchar(50)') AS FirstPersonName
This query uses the XPath to extract the name of the first person and specifies the expected type as nvarchar(50).
How to Use in Practice
When working with XML data, you'll typically follow these steps:
1. Store XML Data: First, store your XML data in a column of type XML or a variable.
2. Query the XML Data: Use query() for retrieving XML fragments and value() for scalar values.
3. Iterate if Necessary: If you need to retrieve multiple values, you might iterate using .nodes() in combination with the aforementioned methods.
Example of Using .nodes()
If you want to extract multiple values from XML:
DECLARE @xmlData XML =
'<root>
<person>
<name>John</name>
<age>30</age>
</person>
<person>
<name>Jane</name>
<age>25</age>
</person>
</root>'
SELECT
T.C.value('(name)[1]', 'nvarchar(50)') AS Name,
T.C.value('(age)[1]', 'int') AS Age
FROM
@xmlData.nodes('/root/person') AS T(C)
In this example, @xmlData.nodes() is used to create a rowset of <person> nodes, allowing you to extract the name and age for each person element.
Conclusion
Using the XML data type methods effectively allows for powerful querying capabilities within SQL Server. Understanding how to leverage query(), value(), and .nodes() is essential for working with XML structured data in your databases.