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Home > Shredding XML docs into relational tables with annotated XML schemas |
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 In this section of DB2 pureXML Cookbook you'll find out how to shred XML documents into relational tables with annotated XML schemas. You'll find examples of the 14 different types of annotations and learn how to define schemas visually in IBM Data Studio.
Table of contents:
11.3 SHREDDING WITH ANNOTATED XML SCHEMAS This section describes another approach to shredding XML documents into relational tables. The approach is called annotated schema shredding or annotated schema decomposition because it is based on annotations in an XML Schema. These annotations define how XML elements and attributes in your XML data map to columns in your relational tables. To perform annotated schema shredding, take the following steps:
Assuming you have defined the relational tables that you want to shred into, let's look at annotating an XML Schema.
Schema annotations are additional elements and attributes in an XML Schema to provide mapping information. DB2 can use this information to shred XML documents to relational tables. The annotations do not change the semantics of the original XML Schema. If a document is valid for the annotated schema then it is also valid for the original schema, and vice versa. You can use an annotated schema to validate XML documents just like the original XML Schema. For an introduction to XML Schemas, see Chapter 16, Managing XML Schemas. The following is one line from an XML Schema:
This line defines an XML element called street and declares that its data type is xs:string and that this element has to occur at least once. You can add a simple annotation to this element definition to indicate that the element should be shredded into the column STREET of the table ADDRESS. The annotation consists of two additional attributes in the element definition, as follows:
The same annotation can also be provided as schema elements instead of attributes, as shown next. You will see later in Figure 11.8 why this can be useful.
< xs:element name="street" type="xs:string" minOccurs="1" > The prefix xs is used for all constructs that belong to the XML Schema language, and the prefix db2-xdb is used for all DB2-specific schema annotations. This provides a clear distinction and ensures that the annotated schema validates the same XML documents as the original schema. There are 14 different types of annotations. They allow you to specify what to shred, where to shred to, how to filter or transform the shredded data, and in which order to execute inserts into the target tables. Table 11.4 provides an overview of the available annotations, broken down into logical groupings by user task. The individual annotations are further described in Table 11.5.
Table 11.4 Overview and Grouping of Schema Annotations
Table 11.5 XML Schema Annotations
To demonstrate annotated schema decomposition we use the shredding scenario in Figure 11.1 as an example. Assume that the target tables have been defined as shown in Figure 11.1. An annotated schema that defines the desired mapping is provided in Figure 11.8. Let's look at the lines that are highlighted in bold font. The first bold line declares the namespace prefix db2-xdb, which is used throughout the schema to distinguish DB2-specific annotations from regular XML Schema tags. The first use of this prefix is in the annotation db2-xdb:defaultSQLSchema, which defines the relational schema of the target tables. The next annotation occurs in the definition of the element name. The two annotation attributes db2-xdb:rowSet="ADDRESS" and db2-xdb:column="NAME" define the target table and column for the name element. Similarly, the street and city elements are also mapped to respective columns of the ADDRESS table. The next two annotations map the phone number and the type attribute to columns in the PHONES table. The last block of annotations belongs to the XML Schema definition of the Cid attribute. Since the Cid attribute value becomes the join key between the ADDRESS and the PHONE table, it has to be mapped to both tables. Two row set mappings are necessary, which requires the use of annotation elements instead of annotation attributes. The first db2-xdb:rowSetMapping maps the Cid attribute to the CID column in the ADDRESS table. The second db2-xdb:rowSet Mapping assigns the Cid attribute to the CID column in the PHONES table.
Figure 11.8 Annotated schema to implement the shredding in Figure 11.1
11.3.2 Defining Schema Annotations Visually in IBM Data Studio
Figure 11.9 Annotated XSD Mapping Editor in Data Studio Developer
11.3.3 Registering an Annotated Schema
Figure 11.10 Registering an annotated XML schema Figure 11.11 shows that you can query the DB2 catalog view syscat.xsrobjects to determine whether a registered schema is enabled for decomposition (Y) or not (N).
Figure 11.11 Checking the status of an annotated XML schema The DECOMPOSITION status of an annotated schema is automatically changed to X (inoperative) and shredding is disabled, if any of the target tables are dropped or a target column is altered. No warning is issued when that happens and subsequent attempts to use the schema for shredding fail. You can also use the following commands to disable and enable an annotated schema for shredding:
ALTER XSROBJECT cust2xsd DISABLE DECOMPOSITION; More on converting XML to relational data:
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