If the specified XPath returns an empty sequence, XMLEXISTS returns false.
如果指定的XPath返回一个空序列，那么XMLEXISTS返回false。
The value produced by that XQuery is either the customer's name element if the predicate is true, or the empty sequence otherwise.
那个XQuery产生的值，当谓词为true时为客户的name元素，否则为空的序列。
This is because, for a given row, the XMLEXISTS predicate evaluates to false only if the XQuery expression inside returns the empty sequence.
这是因为，对于一个给定的行，只有当里面的XQuery表达式返回空序列时，XMLEXISTS谓词才为false。
However, without the square brackets the XQuery expression is a Boolean expression which always returns a Boolean value and never the empty sequence.
然而，如果不使用方括号，XQuery表达式就是一个总是返回boolean值的Boolean表达式，而不会返回空序列。
The XMLEXISTS is a Boolean function that takes an XPath expression as input and returns FALSE if the XPath expression returns an empty sequence and TRUE otherwise.
XMLEXISTS是一个Boolean函数，它接受一个XPath表达式作为输入；如果这个XPath表达式返回空序列，它会返回FALSE，否则返回TRUE。
To fix this, you must declare an XML element containing an empty sequence and add a part to the message with this element. Listing 3 demonstrates how such an XML element can be declared.
为了纠正这个错误，必须声明一个包含空序列的xml元素，并用这个元素将一个部分添加到消息中。
In this case the starting value is returned for an empty sequence, and the function is first applied to the starting value and the first sequence item, then to the result and the next item, and so on.
如果序列是空的，就返回初始值，否则函数会先接收初始值和序列的第一个元素，然后是返回值和下一个元素，依此类推。
In this case the starting value is returned for an empty sequence, and the function is first applied to the starting value and the first sequence item, then to the result and the next item, and so on.
如果序列是空的，就返回初始值，否则函数会先接收初始值和序列的第一个元素，然后是返回值和下一个元素，依此类推。
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