这个方法可以利用关系数据库运算实现，而且不要求预先指定类数。

This method can be implemented by the calculation of the relational data base, and it is not necessary to fix Numbers of classes in advance.

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大部分数据库系统支持时间戳运算。

Most database systems support timestamp arithmetics.

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为了获得这些数据，您可能需要调用 Web 服务、连接到数据库、执行复杂的逻辑运算或从多个数据源汇总数据。

To obtain this information, you may invoke a Web service, connect to a database, perform complex logic, or aggregate data from multiple sources.

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