LINQ to Dataset Left Join，如何计算空值

First, take a look at this thread

首先,看看这个帖子

LINQ to Dataset DBNULL problem / null reference exception

LINQ to Dataset DBNULL问题/ null引用异常

The link above provides you how to LEFT JOIN and then SELECT column that have NULL value in it without taking Exception error.

上面的链接提供了如何LEFT JOIN,然后是SELECT列,其中包含NULL值,而不会出现异常错误。

My problem is I want to COUNT the column is null in WHERE clause (after my LEFT JOIN, some NULL value pop up in the column), something like this:

我的问题是我想在WHERE子句中将COUNT列为null(在我的LEFT JOIN之后,列中弹出一些NULL值),如下所示:

int tmp1 = 
(
from n in dt_query
join m in dt_query2 
on n.Field<string>("VC_CLIENT_MAXID") equals m.Field<string>("VC_CHAT_MAXID") into nm
from m in nm.DefaultIfEmpty()
where 
    //string.IsNullorEmpty(m.Field<string>("VC_CHAT_STAFF"))
    //DBNull.Value.Equals(m.Field<string>("VC_CHAT_STAFF"))
    //m.IsNull("VC_CHAT_STAFF")
    //object.Equals(m.Field<string>("VC_CHAT_STAFF"), xxx) <<< String xxx = null on above
    select n.Field<string>("VC_CLIENT_MAXID")
    ).Count();

The commented lines are the solution I tried but it didn't work for me. Any help would be appreciated.

注释行是我尝试的解决方案,但它对我不起作用。任何帮助,将不胜感激。

2 个解决方案

#1

If what you're asking is how to count the number of records where the joined table is null, then give this a try.

如果您要问的是如何计算连接表为空的记录数,那么请尝试一下。

int tmp1 = 
    (
    from n in dt_query
    join m in dt_query2 
    on n.Field<string>("VC_CLIENT_MAXID") 
        equals m.Field<string>("VC_CHAT_MAXID") into nm

    from LeftJoinM in nm.DefaultIfEmpty()
    where 

        LeftJoinM  == null

    select n.Field<string>("VC_CLIENT_MAXID")
    ).Count();

If I misunderstood you, please let me know in a comment and I'll be happy to update.

如果我误解了你,请在评论中告诉我,我很乐意更新。

#2

I have not tested the following code..Hope this should work

我没有测试下面的代码..希望这应该工作

int tmp1 = 
        (
        from n in dt_query
        join m in dt_query2 
        on n.Field<string>("VC_CLIENT_MAXID") 
            equals m.Field<string>("VC_CHAT_MAXID") into nm

        from temp in nm.where(t => t.Field<string>("VC_CLIENT_MAXID") ==null).DefaultIfEmpty()


        select VC_CLIENT_MAXID=  (temp== null) ? "" :n.Field<string>("VC_CLIENT_MAXID");
        ).Count();

#1