Typed versus Untyped XML
The difference between typed and
untyped XML is simply the process to apply a schema to the XML. For
example, you can create columns, variables, and parameters of type XML
without applying a schema. These instances are untyped. Applying an XML
schema makes the XML instances typed.
Now look at an example. In the following code, an XML data type variable called @var
is declared, and an XML fragment is then assigned the variable. That
variable is then used to insert the XML fragment into the ItemInfo
table. The next statement assigns a different XML fragment to the XML
data type variable and inserts it into the ItemInfo table.
DECLARE @var xml
SET @var =
'<Root>
<Junk1>Some Junk</Junk1>
<Junk2>Some More Junk</Junk2>
<Junk3>Even More Junk</Junk3>
<Junk4>Too Much Junk</Junk4>
</Root>'
INSERT INTO ItemInfo (OrderID, ItemData)
VALUES (1, @var)
SET @var =
'<Vendor Name="Fast Freddys Five Finger Discount" Address="No Specific Location"
Phone="" Description="All conversations should start with pssssst, buddy...." />'
INSERT INTO ItemInfo (OrderID, ItemData)
VALUES (2, @var)
GO
Although this is doable, two important and critical questions should pop into mind:
- How do you validate the XML?
- How do you query the XML?
What this simple example illustrates is that
allowing any format of XML into a column makes it difficult to validate
and even more difficult to query it. For example, suppose you allow
anyone to insert any format of XML into an XML column. This is
unmanageable, and has a negative impact on performance.
Typing XML offers several benefits, the most
important being that the validation constraints are always respected,
and the content of the XML can always be valid as per the schema. Thus,
SQL Servers' query optimizer will always know the structure of the XML,
including data types, and can then generate a more optimized query plan
to query the XML.
With that, now consider how to create and apply XML schemas and use them to validate XML.
XML Schemas
As mentioned earlier, XML validation is
accomplished through the schemas, or XML Schema Definition language.
The XSD language is a specific language used to validate XML documents
(and fragments).
An XML schema “describes” the structure of an XML
document and various constraints on the data in the document. An XML
schema is itself an XML document using the XSD language to describe the
XML document structure, and if you are not familiar with the XSD
language, it can take some time to get used to.
The great thing is that you don't need to learn a
new language because Microsoft has created a nifty little tool to
generate the schema for you based on the XML you want to validate. Walk
through an example.
Open up your favorite text editor (Notepad works just fine) and type in the following XML:
<Order OrderID="1">
<Item>
<ItemNumber>V001</ItemNumber>
<Quantity>1</Quantity>
<Price>299.99</Price>
</Item>
</Order>
Save the text file as orderxml.xml.
With the XML document created, you can use a nifty little tool to
create the schema based on the XML document you just created. The tool
is called the XML Schema Definition Tool (XSD.exe go figure) and
generates XML schema or even Common Language Runtime (CLR) classes from
XDR, XML, and XML files. You can find this tool in the following
location:
C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin\x64
Open a command prompt, and navigate the preceding directory. The syntax for creating the schema is simple:
Xsd.exe filename.xml /outputdirectory /parameters
To create the schema, you need to pass the path
to the file you previously created, and the path and name of the new
XSD schema you want to create. You can optionally supply a number of
parameters.
In your command prompt, type the following (assuming you saved your XML file in C:\Temp):
Xsd.exe c:\temp\orderxml.xml /outputdir:c:\temp
When specifying the output directory, you need to
specify only the directory, as shown in the preceding code. If you
leave off the actual name of the file, it uses the name of the XML file
as the name of the XSD file.
Press the Enter key after you have typed it in,
and quickly you notice a new file in the same directory as your XML
file. Open the orderxml.xsd file
to look at its contents. Intricate syntax isn't covered here, but what
you see is the validation of your XML, and this is what you create your
XML Schema Collection from.
So, copy all the contents of the orderxml.xsd
file to the clipboard, and create a new query window in SQL Server
Management Studio, pasting in the contents of the clipboard. To create
the XML Schema Collection, you simply need to add the CREATE XML SCHEMA COLLECTION statement to the beginning of the schema, as shown here:
CREATE XML SCHEMA COLLECTION OrderInfoSchemaCollection AS
N'<?xml version="1.0" encoding="utf-16"?>
<xs:schema id="NewDataSet" xmlns="" xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
<xs:element name="Order">
<xs:complexType>
<xs:sequence>
<xs:element name="Item" minOccurs="0" maxOccurs="unbounded">
<xs:complexType>
<xs:sequence>
<xs:element name="ItemNumber" type="xs:string" minOccurs="0" />
<xs:element name="Quantity" type="xs:string" minOccurs="0" />
<xs:element name="Price" type="xs:string" minOccurs="0" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
<xs:attribute name="OrderID" type="xs:string" />
</xs:complexType>
</xs:element>
<xs:element name="NewDataSet" msdata:IsDataSet="true"
msdata:UseCurrentLocale="true">
<xs:complexType>
<xs:choice minOccurs="0" maxOccurs="unbounded">
<xs:element ref="Order" />
</xs:choice>
</xs:complexType>
</xs:element>
</xs:schema>’ ;
Now execute the statement to create the schema
collection. A schema is just like any other object in SQL Server, such
as a table or stored procedure. It is an actual object. However, even
though it is created, it still needs to be applied to the XML column of
the ItemInfo table. So, type in the following syntax to alter the
ItemData column of the ItemInfo table.
/* Apply it to the table/columns */
ALTER TABLE ItemInfo
ALTER COLUMN ItemData xml (OrderInfoSchemaCollection)
GO
It didn't work, and you received an error. The
error you received is not because the schema is invalid but because the
two XML fragments inserted earlier can't be validated by the schema.
Remember that you built the schema from an XML document, and that
document is much different from the XML fragments used earlier.
So, you need to delete the records in the ItemInfo table.
DELETE FROM ItemInfo
GO
Now go back and execute the ALTER statement again, and when applied successfully, try to insert some XML. First, try some bogus data.
/* Fail */
DECLARE @var xml
SET @var =
'<Root>
<Junk1>Some Junk</Junk1>
<Junk2>Some More Junk</Junk2>
<Junk3>Even More Junk</Junk3>
<Junk4>Too Much Junk</Junk4>
</Root>'
INSERT INTO ItemInfo (OrderID, ItemData)
VALUES (1, @var)
The preceding code will fail because the XML does
not match, or fails to be validated against the XML schema. The XML is
not valid. So try some valid XML.
/* Succeed */
DECLARE @var xml
SET @var =
'<Order OrderID="1">
<Item>
<ItemNumber>V001</ItemNumber>
<Quantity>1</Quantity>
<Price>299.99</Price>
</Item>
</Order>'
INSERT INTO ItemInfo (OrderID, ItemData)
VALUES (1, @var)
SET @var =
'<Order OrderID="2">
<Item>
<ItemNumber>A017</ItemNumber>
<Quantity>1</Quantity>
<Price>2999.99</Price>
</Item>
</Order>'
INSERT INTO ItemInfo (OrderID, ItemData)
VALUES (2, @var)
GO
The preceding statement executes
successfully because the XML is successfully validated against the
schema and therefore is inserted into the ItemData column.
