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。
The hierarchy of menus has been flattened down until it is only one level deep .
这里菜单只有一级，层次关系扁平化了。
The table contains the flattened union of all possible data columns for the whole hierarchy.
该表包含整个层次结构的所有可能数据列的平展联合。
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