Question

我有一个以下类型的SQL查询，我试图使用LINQ在我的C＃应用程序中获取数据

select 
sum(CASE when outcome = 0 then samplingweight else 0 end) tpcount, 
sum(CASE when outcome = 1 then samplingweight else 0 end) fpcount, 
sum(CASE when outcome = 2 then samplingweight else 0 end) mismatch, 
sum(CASE when outcome = 3 then samplingweight else 0 end) fncount, 
sum(CASE when outcome = 4 then samplingweight else 0 end) tncount, 
sum(samplingweight) totalweight, 
__date, 
scenario from MyTable
where __date = '30-Dec-16 12:00:00 AM' and scenario = 'parcels'
group by __date, scenario

注意：samplingweight是数据库中的双精度值。

我正在尝试并且能够达到以下状态

from pr in this.context.ComputedPrSet
where
    pr.Date.Equals(date) && pr.Scenario.Equals(scenario)
group pr by new { pr.Date, pr.Scenario }
into grp
select new
           {
               grp.Key.Date,
               grp.Key.Scenario,
               TruePositiveCount = grp.Sum(x => x.SamplingWeight) //I am stuck here. Not sure how to aggregate the properties 
           };

注意：上述LINQ查询中的date和scenario被视为执行函数的参数。

有关如何进行的任何想法或提示？

Answer 1

您有两个容易看见的选项：

在求和之前使用.Where()子句过滤grp
在.Sum()

我认为前者更具可读性，而后者更符合您的SQL。根据您的提供商，它们可能会以不同方式转换为SQL。如果性能是个问题，请确保评估生成的SQL。

from pr in this.context.ComputedPrSet
where
    pr.Date.Equals(date) && pr.Scenario.Equals(scenario)
group pr by new { pr.Date, pr.Scenario }
into grp
select new
           {
               grp.Key.Date,
               grp.Key.Scenario,
               TruePositiveCount = grp.Where(x => x.outcome == 0).Sum(x => x.SamplingWeight),
               FPCount = grp.Sum(x => x.outcome == 1 ? x.SamplingWeight : 0),
           };

第三个选项是在数据库中按日期，方案和结果进行分组，然后在代码中重新Select()到您想要的对象中，如下所示：

this.Context.ComputedPrSet
    .Where(pr => pr.Date == date && pr.Scenario == scenario)
    .GroupBy(pr => new { pr.Date, pr.Scenario, pr.Outcome })
    .Select(grp => new {
        grp.Key.Date,
        grp.Key.Scenario,
        grp.Key.Outcome,
        Count = grp.Sum(pr => pr.SamplingWeight)
    })
    .AsEnumerable()
    // .AsEnumerable() will "close" the query in LINQ.
    // Further operations are done via LINQ to Objects after fetching the prior results from your DB
    .GroupBy(e => new { e.Date, e.Scenario })
    .Select(g => new {
        g.Key.Date,
        g.Key.Scenario,
        TruePositiveCount = g.FirstOrDefault(e => e.Outcome == 0),
        FPCount = g.FirstOrDefault(e => e.Outcome == 1),
    });

Answer 2

您可以使用?:运算符执行CASE WHEN部分。

from pr in this.context.ComputedPrSet
where
    pr.Date.Equals(date) && pr.Scenario.Equals(scenario)
group pr by new { pr.Date, pr.Scenario }
into grp
select new
{
    grp.Key.Date,
    grp.Key.Scenario,
    TruePositiveCount = grp.Sum(x => x.Outcome == 0?x.SamplingWeight:0)
};

按LINQ中的聚合函数分组

2 个答案: