Data that you use is often associated with additional information that is not part of the content. To process it in different ways, you can use such metadata to group or classify data.

For example, you might have a collection of digital photographs, and you might associate metadata with each picture, such as information about the photographic characteristics (color composition, focal length) or context (location, kind of subject: landscape, people).

An Oracle XML DB repository resource is an XML document that contains both metadata and data. The data is the contents of element Contents. All other elements in the resource contain metadata. The data of a resource can be XML, but it need not be.

You can associate resources in the Oracle XML DB repository with metadata that you define. In addition to such user-defined metadata, each repository resource also has associated metadata that Oracle XML DB creates automatically and uses (transparently) to manage the resource. Such system-defined metadata includes properties such as the owner and creation date of each resource.

Except for system-defined metadata, you decide which resource information should be treated as data and which should be treated as metadata. For a photo resource, supplemental information about the photo is normally not considered to be part of the photo data, which is a binary image. For text, however, you sometimes have a choice of whether to include particular information in the resource contents (data) or keep it separate and associate it with the contents as metadata — that choice is often influenced by the applications that use or produce the data.

• Kinds of Metadata – Uses of the Term
  The term "metadata" is used in the context of XML in various ways, including XML Schema definitions, XML tags, and Oracle XML DB Repository resource information that supplements the resource content.
• User-Defined Resource Metadata
  User-defined resource metadata is itself represented as XML: it is XML data that is associated with other XML data, describing it or providing supplementary, related information.
• Scenario: Metadata for a Photo Collection
  A scenario used to illustrate the use of schema-based resource metadata uses metadata associated with photographic image files that are stored in repository resources. You can create any number of different kinds of metadata to be associated with the same resource.

Before you can add user metadata to photo resources, you must define the structure of such metadata using XML Schema. An XML schema is created and registered for each kind (technique, category) of photo resource metadata.

BEGIN
  DBMS_XMLSCHEMA.registerSchema(
    SCHEMAURL       => 'imagetechnique.xsd',
    SCHEMADOC       => '<xsd:schema targetNamespace="inamespace"
                                    xmlns:xsd="http://www.w3.org/2001/XMLSchema"
                                    xmlns:xdb="http://xmlns.oracle.com/xdb"
                                    xmlns="inamespace">
                          <xsd:element name="ImgTechMetadata"
                                       xdb:defaultTable="IMGTECHMETADATATABLE">
                            <xsd:complexType>
                              <xsd:sequence>
                                <xsd:element name="Height"      type="xsd:float"/>
                                <xsd:element name="Width"       type="xsd:float"/>
                                <xsd:element name="ColorDepth"  type="xsd:integer"/>
                                <xsd:element name="Title"       type="xsd:string"/>
                                <xsd:element name="Description" type="xsd:string"/>
                              </xsd:sequence>
                            </xsd:complexType>
                          </xsd:element>
                        </xsd:schema>',
    enableHierarchy => DBMS_XMLSCHEMA.ENABLE_HIERARCHY_RESMETADATA);
END;
/

BEGIN
  DBMS_XMLSCHEMA.registerSchema(
    SCHEMAURL       => 'imagecategories.xsd',
    SCHEMADOC       => '<xsd:schema targetNamespace="cnamespace"
                                    xmlns:xsd="http://www.w3.org/2001/XMLSchema"
                                    xmlns:xdb="http://xmlns.oracle.com/xdb"
                                    xmlns="cnamespace">
                          <xsd:element name="ImgCatMetadata"
                                       xdb:defaultTable="IMGCATMETADATATABLE">
                            <xsd:complexType>
                              <xsd:sequence>
                                <xsd:element name="Categories" 
                                             type="CategoriesType"/>
                              </xsd:sequence>
                            </xsd:complexType>
                          </xsd:element>
                          <xsd:complexType name="CategoriesType">
                            <xsd:sequence>
                              <xsd:element name="Category" type="xsd:string"
                                           maxOccurs="unbounded"/>
                            </xsd:sequence>
                          </xsd:complexType>
                        </xsd:schema>',
    enableHierarchy => DBMS_XMLSCHEMA.ENABLE_HIERARCHY_RESMETADATA);
END;
/

You can add, update, and delete user-defined resource metadata using PL/SQL procedures in package DBMS_XDB_REPOS, SQL DML statements INSERT, UPDATE, and DELETE, or WebDAV protocol method PROPPATCH.

You can add, update, and delete user-defined resource metadata in any of the following ways:

• Use PL/SQL procedures in package DBMS_XDB_REPOS:

  • appendResourceMetadata – add metadata to a resource

  • updateResourceMetadata – modify resource metadata

  • deleteResourceMetadata – delete specific metadata from a resource

  • purgeResourceMetadata – delete all metadata from a resource

• Use SQL DML statements INSERT, UPDATE, and DELETE to update the resource directly

• Use WebDAV protocol method PROPPATCH

You use SQL DM statements and WebDAV method PROPPATCH to update or delete metadata in the same way as you add metadata. If you supply a complete Resource element for one of these operations, then keep in mind that each resource metadata property must be a child (not just a descendant) of element Resource — if you want multiple metadata elements of the same kind, you must collect them as children of a single parent metadata element. The order among such top-level user-defined resource metadata properties is unimportant and is not necessarily maintained by Oracle XML DB.

The separate PL/SQL procedures in package DBMS_XDB_REPOS are similar in their use. Each can be used with either XML schema-based or non-schema-based metadata. Some forms (signatures) of some of the procedures apply only to schema-based metadata. Procedures appendResourceMetadata and deleteResourceMetadata are illustrated here with examples.

• Adding Metadata Using APPENDRESOURCEMETADATA
  You can use procedure DBMS_XDB_REPOS.appendResourceMetadata to add user-defined metadata to resources.
• Deleting Metadata Using DELETERESOURCEMETADATA
  You can use procedure DBMS_XDB_REPOS.deleteResourceMetadata to delete specific metadata associated with a resource. To delete all of the metadata associated with a resource, you can use procedure DBMS_XDB_REPOS.purgeResourceMetadata.
• Adding Metadata Using SQL DML
  An alternative to using procedure DBMS_XDB_REPOS.appendResourceMetadata to add, update, or delete resource metadata is to update the RESOURCE_VIEW directly using DML statements INSERT and UPDATE.
• Adding Metadata Using WebDAV PROPPATCH
  An alternative to using procedure DBMS_XDB_REPOS.appendResourceMetadata to add resource metadata is to use WebDAV method PROPPATCH.

You can use metadata column RESID when querying resource metadata, to join the metadata with the associated data.

SELECT ANY_PATH
  FROM RESOURCE_VIEW rs, imgcatmetadatatable ct, imgtechmetadatatable tt
  WHERE XMLExists(
          'declare namespace r
             = "http://xmlns.oracle.com/xdb/XDBResource.xsd"; (: :)
           declare namespace c
             = "cnamespace"; (: :)
           /r:Resource/c:ImgCatMetadata/Categories/Category[text()="Vacation"]'
          PASSING RES)
    AND XMLExists(
          'declare namespace r
             = "http://xmlns.oracle.com/xdb/XDBResource.xsd"; (: :)
           declare namespace i
             = "inamespace"; (: :)
           /r:Resource/i:ImgTechMetadata/Title[text()="Pig Riding Horse"]'
          PASSING RES)
    AND rs.RESID = ct.RESID
    AND rs.RESID = tt.RESID;

ANY_PATH
--------------------------
/public/horse_with_pig.jpg
 
1 row selected.

Digital cameras include image metadata as part of the image files they produce.

• EXIF – Exchangeable Image File Format

• IPTC-NAA IIM – International Press Telecommunications Council-Newspaper Association of America Information Interchange Model

• XMP – Extensible Metadata Platform

EXIF is the metadata standard for digital still cameras. EXIF metadata is stored in TIFF and JPEG image files. IPTC and XMP metadata is commonly embedded in image files by desktop image-processing software.

You store user-defined resource metadata that is not XML Schema-based as a CLOB instance under the Resource element of the associated resource.

     <Resource xmlns="http://xmlns.oracle.com/xdb/XDBResource.xsd"
       <Owner>DESELBY</Owner>
       ... <!-- other system-defined metadata -->
       <!-- contents of the resource>
       <Contents>
         ...
       </Contents>
       <!-- User-defined metadata (appearing within different namespace) -->
       <MyOwnMetadata xmlns="http://www.example.com/custommetadata">
         <MyElement1>value1</MyElement1>
         <MyElement2>value2</MyElement2>
       </MyOwnMetadata>
     </Resource>

DECLARE
  res BOOLEAN;
BEGIN
  res := DBMS_XDB_REPOS.createResource(
           '/public/NurseryRhyme.txt',
           bfilename('MYDIR', 'tdadxdb-xdb_repos_meta-011.txt'),
           nls_charset_id('AL32UTF8'));
  UPDATE RESOURCE_VIEW SET RES = 
   XMLQuery('declare namespace r = "http://xmlns.oracle.com/xdb/XDBResource.xsd"; (: :)
             declare namespace n = "nurserynamespace"; (: :)
             copy $tmp := . modify insert node
              <n:NurseryMetadata>
                <Author>Mother Goose</Author>
              </n:NurseryMetadata>
             into $tmp/r:Resource
             return $tmp'
            PASSING RES
            RETURNING CONTENT)
   WHERE equals_path(RES, '/public/NurseryRhyme.txt') = 1;
END;
/

PL/SQL procedure successfully completed.
 

SELECT XMLSerialize(DOCUMENT rs.RES AS CLOB) FROM RESOURCE_VIEW rs
  WHERE equals_path(RES, '/public/NurseryRhyme.txt') = 1;
 
XMLSERIALIZE(DOCUMENTRS.RESASCLOB)
--------------------------------------------------------------------------------
<Resource xmlns="http://xmlns.oracle.com/xdb/XDBResource.xsd" Hidden="false" Inv
alid="false" Container="false" CustomRslv="false" VersionHistory="false" StickyR
ef="true">
  <CreationDate>2005-05-24T13:51:48.043234</CreationDate>
  <ModificationDate>2005-05-24T13:51:48.290144</ModificationDate>
  <DisplayName>NurseryRhyme.txt</DisplayName>
  <Language>en-US</Language>
  <CharacterSet>UTF-8</CharacterSet>
  <ContentType>text/plain</ContentType>
  <RefCount>1</RefCount>
  <ACL>
    <acl description="Public:All privileges to PUBLIC" xmlns="http://xmlns.oracl
e.com/xdb/acl.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:sch
emaLocation="http://xmlns.oracle.com/xdb/acl.xsd                           http:
//xmlns.oracle.com/xdb/acl.xsd" shared="true">
      <ace>
        <principal>PUBLIC</principal>
        <grant>true</grant>
        <privilege>
          <all/>
        </privilege>
      </ace>
    </acl>
  </ACL>
  <Owner>TDADXDB_META</Owner>
  <Creator>TDADXDB_META</Creator>
  <LastModifier>TDADXDB_META</LastModifier>
  <SchemaElement>http://xmlns.oracle.com/xdb/XDBSchema.xsd#text</SchemaElement>
  <Contents>
    <text>Mary had a little lamb
Its fleece was white as snow
and everywhere that Mary went
that lamb was sure to go
</text>
  </Contents>
  <n:NurseryMetadata xmlns:n="nurserynamespace">
    <Author xmlns="">Mother Goose</Author>
  </n:NurseryMetadata>
</Resource>
 
1 row selected.

You can use PL/SQL procedures DBMS_XMLSCHEMA.registerSchema, DBMS_XDBZ.enable_hierarchy, DBMS_XDBZ.disable_hierarchy, DBMS_XDBZ.is_hierarchy_enabled, DBMS_XDB_REPOS.appendResourceMetadata, DBMS_XDB_REPOS.deleteResourceMetadata, DBMS_XDB_REPOS.purgeResourceMetadata, and DBMS_XDB_REPOS.updateResourceMetadata to perform resource metadata operations.

• DBMS_XMLSCHEMA.registerSchema – Register an XML schema. Parameter ENABLEHIERARCHY affects resource metadata.

• DBMS_XDBZ.enable_hierarchy – Enable repository support for an XMLType table or view. Use parameter HIERARCHY_TYPE with a value of DBMS_XDBZ.ENABLE_HIERARCHY_RESMETADATA to enable resource metadata. This adds column RESID to track the resource associated with the metadata.

• DBMS_XDBZ.disable_hierarchy – Disable all repository support for an XMLType table or view.

• DBMS_XDBZ.is_hierarchy_enabled – Tests, using parameter HIERARCHY_TYPE, whether the specified type of hierarchy is currently enabled for the specified XMLType table or view. Value DBMS_XDBZ.IS_ENABLED_RESMETADATA for HIERARCHY_TYPE tests whether resource metadata is enabled.

• DBMS_XDB_REPOS.appendResourceMetadata – Add metadata to a resource.

• DBMS_XDB_REPOS.deleteResourceMetadata – Delete specified metadata from a resource.

• DBMS_XDB_REPOS.purgeResourceMetadata – Delete all user-defined metadata from a resource. For schema-based resources, optional parameter DELETE_OPTION can be used to specify whether or not to delete the metadata information, in addition to unlinking it.

• DBMS_XDB_REPOS.updateResourceMetadata – Update the metadata for a resource.