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.
如果检索到的列值为SQLNULL，那么不会给此类值分配任何XPath变量引用表达式。
Note that the syntax of using the $treeFarm context attribute reference is identical to the $treeList variable reference examined earlier.
要注意，使用 $treeFarm上下文属性引用的方法和前面分析的 $treeList变量引用一致。
Therefore flow developers should not use routing decisions in which variable reference expressions map onto name columns whose values may be NULL.
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。
If you don't declare a tunnel parameter because it is not used in that template, then you don't have to worry about its name clashing with any variable reference in that template.
如果没有在模板中声明通道参数，而模板正好又没使用它的话，就不用担心和模板中的变量引用发生名称冲突。
If you don't declare a tunnel parameter because it is not used in that template, then you don't have to worry about its name clashing with any variable reference in that template.
如果没有在模板中声明通道参数，而模板正好又没使用它的话，就不用担心和模板中的变量引用发生名称冲突。
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