Linq：计数行组，包括 0 表示缺失的行

Question

我想使用 Linq to Entities 将活动列表总结为上周按天计算的出现次数。例如，假设我的数据库中有这样的数据：

Id | Date
1  | 2011-09-30 0:00:0
2  | 2011-10-02 0:00:00  
3  | 2011-10-02 0:00:00
4  | 2011-10-02 0:00:00
5  | 2011-10-04 0:00:00
6  | 2011-10-04 1:30:30
7  | 2011-10-04 0:00:00
8  | 2011-10-06 0:00:00

假设今天的日期是 2011-10-07

我想生成以下内容：

Date        | Count
2011-10-01  | 0
2011-10-02  | 3
2011-10-03  | 0
2011-10-04  | 3
2011-10-05  | 0
2011-10-06  | 1
2011-10-07  | 0

这是我可以用来按日期对出现的事件进行分组的示例技术，但我缺少零。

// Using Linq to Objects for demonstration purpose only
var activities = new List<Activity>();
activities.Add(new Activity { Id = 1, Date = new DateTime(2011, 9, 30)});
activities.Add(new Activity { Id = 2, Date = new DateTime(2011, 10, 2)});
activities.Add(new Activity { Id = 3, Date = new DateTime(2011, 10, 2)});
activities.Add(new Activity { Id = 4, Date = new DateTime(2011, 10, 2)});
activities.Add(new Activity { Id = 5, Date = new DateTime(2011, 10, 4)});
activities.Add(new Activity { Id = 6, Date = new DateTime(2011, 10, 4, 1, 30, 30) });
activities.Add(new Activity { Id = 7, Date = new DateTime(2011, 10, 4)});
activities.Add(new Activity { Id = 8, Date = new DateTime(2011, 10, 6)});

var data = (from a in activities
            group a by a.Date.Date into g
            where g.Key > DateTime.UtcNow.AddDays(-7)
            select new {
                Date = g.Key,
                Count = g.Count()
            }).ToList();

结果如下：

Date        | Count
2011-10-02  | 3
2011-10-04  | 3
2011-10-06  | 1

任何人都知道如何使用 Linq to Entities 包含丢失的零？一旦将结果存入内存，我总是可以枚举结果，但是直接从数据库中获取它会很好。

Answer 1

使用循环生成可能同样容易，因为数据在数据库中尚不可用，并且通过 LINQ 生成它会很复杂：

var firstDate = data.Min(d => d.Date).AddDays(-1);
var lastDate = data.Max(d => d.Date).AddDays(1);

for (var date = firstDate; date <= lastDate; date = date.AddDays(1))
    if (!data.Exists(d => d.Date == date))
        data.Add(new { Date = date, Count = 0 });

data = data.OrderBy(d => d.Date);
// do something with data

除非您的数据集非常非常大，否则与数据库请求所需的时间相比，内存中的查询和列表调整将是微不足道的。