• Article

Requesting URL References to BLOB Data Using sql:encode (SQLXML 4.0)

In an annotated XSD schema, when an attribute (or element) is mapped to a BLOB column in Microsoft SQL Server, the data is returned in Base 64-encoded format within XML.

If you want a reference to the data (a URI) to be returned that can be used later to retrieve the BLOB data in a binary format, specify the sql:encode annotation. You can specify sql:encode on an attribute or element of simple type.

Specify the sql:encode annotation to indicate that a URL to the field should be returned instead of the value of the field. sql:encode depends on the primary key to generate a singleton select in the URL. The primary key can be specified using the sql:key-fields annotation.

The sql:encode annotation can be assigned the "url" or the "default" value. A value of "default" returns data in Base 64-encoded format.

The sql:encode annotation cannot be used with sql:use-cdata or on the ID, IDREF, IDREFS, NMTOKEN, or NMTOKENS attribute types. It can also not be used with XSD fixed attribute.

Note

BLOB-type columns cannot be used as a part of a key or foreign key.

Examples

To create working samples using the following examples, you must meet certain requirements. For more information, see Requirements for Running SQLXML Examples.

A. Specifying sql:encode to obtain a URL reference to BLOB data

In this example, the mapping schema specifies sql:encode on the LargePhoto attribute to retrieve the URI reference to a specific product photo (instead of retrieving the binary data in Base 64-encoded format).

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            xmlns:sql="urn:schemas-microsoft-com:mapping-schema">

  <xsd:element name="ProductPhoto" sql:relation="Production.ProductPhoto" 
               sql:key-fields="ProductPhotoID" >
   <xsd:complexType>
      <xsd:attribute name="ProductPhotoID"  type="xsd:int"  />
     <xsd:attribute name="LargePhoto" type="xsd:string" sql:encode="url" />
    </xsd:complexType>
  </xsd:element>
</xsd:schema>

To test a sample XPath query against the schema

  1. Copy the schema code above and paste it into a text file. Save the file as sqlEncode.xml.

  2. Copy the following template and paste it into a text file. Save the file as sqlEncodeT.xml in the same directory where you saved sqlEncode.xml.

    <ROOT xmlns:sql="urn:schemas-microsoft-com:xml-sql">
    <sql:xpath-query mapping-schema="sqlEncode.xml">
        /ProductPhoto[@ProductPhotoID=100]
    </sql:xpath-query>
</ROOT>

    The directory path specified for the mapping schema (sqlEncode.xml) is relative to the directory where the template is saved. An absolute path also can be specified, for example:

    mapping-schema="C:\SqlXmlTest\sqlEncode.xml"

  3. Create and use the SQLXML 4.0 Test Script (Sqlxml4test.vbs) to execute the template.

    For more information, see Using ADO to Execute SQLXML 4.0 Queries.

This is the result:

<ROOT xmlns:sql="urn:schemas-microsoft-com:xml-sql">
   <ProductPhoto ProductPhotoID="100"
                 LargePhoto="dbobject/Production.ProductPhoto[@ProductPhotoID="100"]/@LargePhoto" /> 
</ROOT>