如何在LINQ中执行嵌套的Join,Add和Group？

Question

我有两个目标,让我们给他们打电话Input和Output

Input有属性Input_ID,Label并且有属性和Input_Amt
OutputOutput_IDOutput_Amt

我想在LINQ中执行等效的SQL语句:

SELECT Label, Sum(Added_Amount) as Amount FROM
    (SELECT I.Label, I.Input_Amt + ISNULL(O.Output_Amt, 0) as Added_Amount
    FROM Input I LEFT OUTER JOIN Output O ON I.Input_ID = O.Output_ID)
GROUP BY Label

对于内部查询,我写的是:

var InnerQuery = from i in input
                 join o in output
                 on i.Input_ID equals o.Output_ID into joined
                 from leftjoin in joined.DefaultIfEmpty()
                 select new
                 {
                     Label = i.Label,
                     AddedAmount = (i.Input_Amt + leftjoin.Output_Amt)
                 };

但是,在测试中,语句返回null.是什么赋予了？

另外,在我将所有金额添加到一起后,如何在单个LINQ语句中继续所需的查询并执行组？

Answer 1

好的,既然我明白了什么会好一些,那么主要的问题是你没有得到相当于ISNULL的位.试试这个:

var InnerQuery = from i in input
                 join o in output
                 on i.Input_ID equals o.Output_ID into joined
                 from leftjoin in joined.DefaultIfEmpty()
                 select new
                 {
                     Label = i.Label,
                     AddedAmount = (i.Input_Amt + (leftjoin == null ? 0 : leftjoin.Output_Amt))
                 };