现在，看一下包含可选元素的XML数据。
Now, look at the XML data that contains an optional element.
指定对于有效的交换，哪些数据元素是必需的，哪些是可选的(基数)？
The specifications of data elements that are required and optional for a valid exchange (the cardinality)?
<xf:submission>元素支持几种可选的属性来帮助控制XForms 处理程序从数据模型实例生成的XML
The <xf:submission> element allows several optional attributes to help control the XML created out of the data model instance by the XForms processor
这个问题就是在关系数据库中表示可选x ml元素。
This is the problem of representing optional XML elements in relational databases.
首先，整个过程背后的逻辑是合理的：如果您将这项数据元素从可选改为必需，那么您基本上就是复制了系统中其他地方所使用的错误处理过程。
At first, the logic behind this sounds reasonable: If you change this data element from optional to required, you essentially duplicate error processing that occurs elsewhere in the system.
这些规则定义哪些元素是有效的、文档中元素的次序、属性的数据类型、一个元素是必需的还是可选的等等。
These rules describe, for example, which elements are valid, the order of elements within a document, the data types of attributes, whether an element is required or optional, and so on.
接下来的部分比较有趣：COMMENT数据库列，它代表可选的XML元素。
Next comes the interesting part: the COMMENT database column, which represents the optional XML element.
可选的数据元素用虚线表示，必需的数据元素用实线表示。
Dashed lines are used for optional data elements and solid lines are used for required data elements.
但该元素作为可选元素添加，以便没有cell - phone元素的XML数据也可以成功通过验证。
But it is added as optional so XML data without the cell-phone element is also validated successfully.
最后，Atom为核心名称空间中的各种必需和可选元数据元素提供了严格定义。
Finally, Atom provides rigorous definitions for the various required and optional metadata elements within its core namespace.
定义了数据的哪些节是必须的或者是可选的，数据节的数量和数据元素的顺序。
Electronic data Interchange for Administration, Commerce and Transport (EDIFACT) defines which data segments are mandatory or optional, and the number and order of elements.
定义了数据的哪些节是必须的或者是可选的，数据节的数量和数据元素的顺序。
Electronic data Interchange for Administration, Commerce and Transport (EDIFACT) defines which data segments are mandatory or optional, and the number and order of elements.
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