c# - 按值过滤 linq 查询

Question

如果我有以下查询:-

var Values = from data in DtSet.Tables["tblCosts"].AsEnumerable()
            group data by new
            {
                InvNo = data.Field<double>("InvoiceNo"),
                AccRef = data.Field<double>("SiteRefNum"),
            }
            into g
            select new
            {
                Code = "1",
                InvType = "I",
                Account = g.Key.AccRef,
                InvNo = g.Key.InvNo,
                ChargeTotal = g.Sum(d => d.field<double>("Charge")
            };

...排除 ChargeTotal 列之和为零的行的最佳方法是什么？我假设我必须使用 WHERE，但我不确定将它放在哪里。

Answer 1

你没有很多解决方案，因为 where 必须在 group by 之后调用(这取决于组)，然后:

var Values = from data in DtSet.Tables["tblCosts"].AsEnumerable()
             group data by new
             {
                 InvNo = data.Field<double>("InvoiceNo"),
                 AccRef = data.Field<double>("SiteRefNum"),
             }
             into g
             let sum = g.Sum(d => d.field<double>("Charge"))  // Sum only computed once
             where sum != 0
             select new
             {
                 Code = "1",
                 InvType = "I",
                 Account = g.Key.AccRef,
                 InvNo = g.Key.InvNo,
                 ChargeTotal = sum
             };

要稍微提高性能(你必须测试两者以查看是否有任何性能改进，因为它高度依赖于数据)，如果 d.field("Charge") 始终 >= 0，你可以这样做:

var Values = from data in DtSet.Tables["tblCosts"].AsEnumerable()
             group data by new
             {
                 InvNo = data.Field<double>("InvoiceNo"),
                 AccRef = data.Field<double>("SiteRefNum"),
             }
             into g
             where g.Any(d => d.field<double>("Charge") != 0)  
             select new
             {
                 Code = "1",
                 InvType = "I",
                 Account = g.Key.AccRef,
                 InvNo = g.Key.InvNo,
                 ChargeTotal = g.Sum(d => d.field<double>("Charge"))
             };