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。
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。
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