SQL Server的XQUERY返回NULL，而不是空 [英] SQL Server xQuery return NULL instead of empty

问题描述

因此​​，在这个例子中，我试图返回一个空nvarchar的一个​​NULL，而不是为元素1和元素3。我似乎无法找到一个答案，这在任何地方，或者如果它甚至有可能。我知道我可以检查.exists（）与案例/当一个NULLIF，但我宁愿不作出这样的检查，为了表现。

 WqlEventQuery查询=新WqlEventQuery（）;
        query.EventClassName =__InstanceCreationEvent;
        query.Condition =TargetInstance ISA'Win32_NTLogEvent';
        query.GroupWithinInterval =新时间跨度（0,0，10）;
        System.Collections.Specialized.StringCollection集合=
            新System.Collections.Specialized.StringCollection（）;
        collection.Add（TargetInstance.SourceName）;
        query.GroupByPropertyList =收集;
        query.HavingCondition =NumberOfEvents> 25;

解决方案

我不知道你是从你的问题写什么语言，但SQL Server有一个 NULLIF（VAL，校验值）功能。如果你可以包括在SQL传递给了应该做的伎俩服务器：

 选择NULLIF（元素1，''）
 

将返回一个空如果元素1等于空字符串（''）

So in this example I'm trying to return a NULL instead of an empty nvarchar for element2 and element3. I can't seem to find an answer to this anywhere, or if it's even possible. I know I can check .exists() with a nullif of Case/When, but I'd rather not make that check for the sake of performance.

 WqlEventQuery query = new WqlEventQuery();
        query.EventClassName = "__InstanceCreationEvent";
        query.Condition = "TargetInstance ISA 'Win32_NTLogEvent'";
        query.GroupWithinInterval = new TimeSpan(0, 0, 10);
        System.Collections.Specialized.StringCollection collection =
            new System.Collections.Specialized.StringCollection();
        collection.Add("TargetInstance.SourceName");
        query.GroupByPropertyList = collection;
        query.HavingCondition = "NumberOfEvents > 25";

解决方案

I'm not sure what language you are writing in from your question, but SQL Server has a NullIf(val, CheckValue) function. If you can include that in the sql you pass to the server that should do the trick:

   Select NullIf(element2, '')

will return a null if element2 is equal to an empty string ('')

这篇关于SQL Server的XQUERY返回NULL，而不是空的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！