c# - linq选择总和和平均值进入 View 模型

Question

如何在单个选择语句中执行此操作？

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

您可以按常量分组，以便获得总和和平均值。然后使用 SingleAsync 获取单个结果。

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();

另一种选择是让您的异步数据库调用并行运行

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,
};

如果非要我猜的话，我会说单个数据库调用的性能会更好，但您始终可以测试这两个选项，看看哪个更好。