linq选择sum和average进入视图模型

Question

如何在单个select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

您可以按常数分组，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();

另一种选择是实际让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,
};

如果我不得不猜测我说单个数据库调用会表现更好，但你总是可以测试两个选项，看看哪个更好。