string xQuery simple if not empty 条件
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xQuery simple if not empty conditional
提问by Moak
I'm trying to learn xQuery coming from a php background, I have this expression working as expected
我正在尝试从 php 背景学习 xQuery，我让这个表达式按预期工作
<![CDATA[
declare variable $doc as node() external;
declare variable $id external;
let $cont := data($doc//div[@class="cont"])
let $title := data($doc//p[@class="vtitle"])
let $text := data($doc//div[@class="venue-cont-left"])
return
<venue id="{$id}">
<title>{$title}</title>
<text>{$text}</text>
</venue>
]]>
However now I simply want to make a check weather $title is empty or not
但是现在我只想检查天气 $title 是否为空
<![CDATA[
declare variable $doc as node() external;
declare variable $id external;
if(empty(data($doc//p[@class="vtitle"]))) then
(
let $id :=$id
return
<venue id="{$id}" />
) else (
let $cont := data($doc//div[@class="cont"])
let $title := data($doc//p[@class="vtitle"])
let $text := data($doc//div[@class="venue-cont-left"])
return
<venue id="{$id}">
<title>{$title}</title>
<text>{$text}</text>
</venue>
)
]]>
This does not work because i get this resulting output
这不起作用，因为我得到了这个结果输出
<venue id="4">
<title/>
<text>
PHONE:
ADDRESS:....
As you see venue 4 has no title so it should have been returned as <venue id="4" />
如您所见，场地 4 没有标题，因此它应该作为 <venue id="4" />
Thanks for any help!
谢谢你的帮助！
回答by
Use:
用：
declare variable $doc as node() external;
declare variable $id external;
declare variable $title := data($doc//p[@class="vtitle"]);
<venue id="{$id}">{
if ($title)
then <title>{$title}</title>
else (),
<text>{data($doc//div[@class="venue-cont-left"])}</text>
}</venue>
Note: Empty sequence efective boolean value is false.
注意：空序列有效布尔值为假。
回答by Dennis Münkle
There is only one scenario I can think of where your query fails: If your XML contains a pnode like this with no content:
我只能想到一种情况，您的查询失败的地方：如果您的 XML 包含这样一个p没有内容的节点：
<p class="vtitle" />
With this, the following code snippet returns a zero-length string ""(not an empty sequence):
有了这个，下面的代码片段返回一个零长度的字符串""（不是一个空序列）：
data($doc//p[@class="vtitle"])
The problem here is that the function empty()checks for an empty sequence. Therefore, empty("")returns false.
这里的问题是该函数empty()检查空序列。因此，empty("")返回false。
If you would leave away the empty()and switch the then and else expressions your code should work, because then the Effective Boolean Value (EBV) is processed. And, the EBV of an empty string as well as of an empty sequence is false.
如果您放弃 the empty()and switch then 和 else 表达式，您的代码应该可以工作，因为随后会处理有效布尔值 (EBV)。并且，空字符串和空序列的 EBV 是false。
Hope that makes sense?
希望这是有道理的？
