重要的是，这是一个递归定义：有时属性将映射成零或者多列。

The important thing is that this is a recursive definition: at some point the attribute will be mapped to zero or more columns.

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类属性将映射成关系数据库中的零或几列。

A class attribute will map to either zero or a number of columns in a relational database.

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清单 20 中的函数返回矢量的矢量，这是因为每个文档都可能生成零到多个映射条目，而每个映射条目都被表示为一个矢量。

The function in Listing 20 returns a vector of vectors. This is because each document could generate zero to many map entries, and each map entry is represented as a vector.

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