linq select sum and average into view model

Question

如何在單個select語句中執行此操作？

var data = new CampaignData() 
{ 
    TotalPost = await context.Campaigns.SumAsync(c => c.Posts), 
    AveragePost = await context.Campaigns.AverageAsync(c => c.Posts), 
    TotalImpression = await context.Campaigns.SumAsync(c => c.Impressions), 
    AverageImpressions = await context.Campaigns.AverageAsync(c => c.Impressions), 
}; 

Answer 1

您可以按常數分組，以便獲得總和和平均值。然後使用SingleAsync來獲得單一結果。

var data = await (from compaign in context.Compaigns 
        group compaign by 1 into grp 
        select new CampaignData() 
        { 
         TotalPost = grp.Sum(cc => cc.Posts), 
         AveragePost = grp.Average(c => c.Posts), 
         TotalImpression = grp.Sum(c => c.Impressions), 
         AverageImpressions = grp.Average(c => c.Impressions), 
        }).SingleAsync(); 

另一種選擇是真正讓你的異步調用數據庫中運行並行

var totalPostTask = context.Campaigns.SumAsync(c => c.Posts); 
var averagePostTask = context.Campaigns.AverageAsync(c => c.Posts); 
var totalImpressionTask = context.Campaigns.SumAsync(c => c.Impressions); 
var averageImpressionsTask = context.Campaigns.AverageAsync(c => c.Impressions); 

await Task.WhenAll(
    totalPostTask, 
    averagePostTask, 
    totalImpressionTask, 
    averageImpressionsTask); 

var data = new CampaignData() 
{ 
    TotalPost = totalPostTask.Result, 
    AveragePost = averagePostTask.Result, 
    TotalImpression = totalImpressionTask.Result, 
    AverageImpressions = averageImpressionsTask.Result, 
}; 

如果我猜我想說的單一數據庫調用會表現得更好，但你隨時可以測試都選擇出去看哪個更好。