yacc允许您通过符号名引用表达式的组成部分。

Yacc allows you to refer to the components of an expression by symbolic names.

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因此，流开发人员不应该使用变量引用表达式映射到名称列可能为 NULL 的路由决策。

Therefore flow developers should not use routing decisions in which variable reference expressions map onto name columns whose values may be NULL.

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如果检索到的列值为SQL NULL，那么不会给此类值分配任何XPath变量引用表达式。

In the case where a retrieved column has a value of SQL NULL, then no XPath variable reference expression is assigned to such a value.

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