Flattened hierarchy: Relational databases represent data in the form of tables.
扁平的层次结构(Flattened hierarchy)：关系数据库以表的形式来表示数据。
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Another benefit of having object wrappers is that they can present data stored in relational tables in a natural object hierarchy that captures the relationship between the data.
The pureXML feature of DB2 9 has revolutionized support for XML data-handling XML as a new data type that is stored in a natural hierarchy-different from relational data.