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SQL/XML Mapping Rules

The XML publishing functions use SQL values to create XML values, and these XML values have W3C XML Schema types. When we discussed the XML publishing functions, we did not address specifically how the XML representation is determined. The mapping rules of SQL/XML describe in excruciating detail how SQL values can be mapped to and from XML values, and how SQL metadata can be mapped to and from W3C XML Schemas. To give a flavor for the level of detail in which this is specified, here are the equivalent headings from the SQL/XML specification's table of contents:

• Mapping SQL character sets to Unicode.
  
• Mapping SQL <identifier>s to XML Names.
  
• Mapping SQL data types (as used in SQL-schemas to define SQL-schema objects such as columns) to XML Schema data types.
  
• Mapping values of SQL data types to values of XML Schema data types.
  
• Mapping an SQL table to an XML document and an XML Schema document.
  
• Mapping an SQL schema to an XML document and an XML Schema document.
  
• Mapping an SQL catalog to an XML document and an XML Schema document.
  
• Mapping Unicode to SQL character sets.
  
• Mapping XML Names to SQL <identifier>s.
  

These mappings can be parameterized in several ways, including the target namespace for the result, whether to handle nulls using xsi:nil or absence, and whether to map a table to a single element or a forest of elements. Here is an XML representation of the Customers table shown earlier, using a single element for each table and no target namespace:

Here is an XML representation of the same table using a forest of elements to represent each table:

These mappings are also defined on the metadata level. For instance, SQL/XML defines how the datatypes of SQL are represented in the equivalent XML Schema. Each SQL type is derived from an equivalent built-in W3C XML Schema type. Where needed, facets are used to represent constraints added to those of the base type:

As mentioned above, there are two ways to represent null values. Suppose the City column may have null values. Here is a row in the Customer's table that represents a null value using the first strategy, a nilled element:

Here is a row that uses the second strategy, an absent element: