XML Schema Complete Reference, The

24.17. extension Element

The extension element is used to derive a complexType from another by adding attribute types for simple content and element types for complex content. A database designer has two major options in designing a schema to create an extension:

• Add a type column to a table and handle the extensions in check clauses and triggers that key off of the type column. In other words, add the type to the existing set of tables.

• Create a new table (or tables) to handle the new type.

Each of these approaches has trade-offs. The first approach is best when the extension element type is associated with content that is mapped to a couple of database columns and not relationships. It best handles the simple cases of extension element types. ...