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SQL NULL，那么不会给此类值分配任何XPath变量引用表达式。

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SQL USES a small integer which contains a zero if the column is not null or -1 if the column contains nulls.

如果列不包含空值，则SQL使用一个包含一个0的小整数；反之则包含- 1。

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The null indicator values are populated during the execution of the SQL FETCH statement.

空指示符值在 SQL FETCH 语句执行过程中填充。

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