Linq - 按日期分组并选择计数

Question

我目前正在解决一个问题,我想运行一个查询,按所选日期对结果进行分组.

对于这个例子,想象一个像这样的简单模型:

public class User
{
      public DateTime LastLogIn {get; set;}
      public string Name {get; set;}
}

我正在寻找的解决方案是获取按日期登录的用户数.在数据库中,DateTime与日期和时间组件一起存储,但对于此查询,我真的只关心日期.

我现在拥有的是:

    context.Users
            .Where((x.LastLogIn  >= lastWeek)    
                && (x.LastLogIn <= DateTime.Now))
            .GroupBy(x => EntityFunctions.TruncateTime(x.LastLogIn))
            .Select(x => new
            {
                Value = x.Count(),
                Day = (DateTime)EntityFunctions.TruncateTime(x.Key)
            }).ToList();

然而,上面返回一个空列表.

最终目标是拥有一个对象列表,其中包含一个值(一天登录的用户数)和一天(相关日期)

有什么想法吗？

将查询更改为:

    context.Users
            .Where((x.LastLogIn  >= lastWeek)    
                && (x.LastLogIn <= DateTime.Now))
            .GroupBy(x => EntityFunctions.TruncateTime(x.LastLogIn))
            .Select(x => new
            {
                Value = x.Count(),
                Day = (DateTime)x.Key
            }).ToList();

它现在返回一个包含单个项目的列表,其中Value是与where子句匹配的Users总数,而Day是第一天.它似乎仍然没有能够分组

注意:结果是上面的代码是对的,我只是做了别的错误.

它生成的Sql是(注意这里可能是非常轻微的语法错误,我为此调整它):

SELECT 
1 AS [C1], 
[GroupBy1].[A1] AS [C2], 
 CAST( [GroupBy1].[K1] AS datetime2) AS [C3]
FROM ( SELECT 
        [Filter1].[K1] AS [K1], 
        COUNT([Filter1].[A1]) AS [A1]
        FROM ( SELECT 
                 convert (datetime2, convert(varchar(255), [Extent1].[LastLogIn], 102) ,  102) AS [K1], 
                1 AS [A1]
                FROM [dbo].[Users] AS [Extent1]
                WHERE (([Extent1].[LastLogIn] >= @p__linq__1) AND ([Extent1].[LastLogIn] <= @p__linq__2)
        )  AS [Filter1]
       GROUP BY [K1]
)  AS [GroupBy1] 

Answer 1

你不需要第二个TruncateTime:

context.Users
    .Where((x.LastLogIn  >= lastWeek) && (x.LastLogIn <= DateTime.Now))
    .GroupBy(x => DbFunctions.TruncateTime(x.LastLogIn))
    .Select(x => new
    {
        Value = x.Count(),
        // Replace the commented line
        //Day = (DateTime)DbFunctions.TruncateTime(x.Key)
        // ...with this line
        Day = (DateTime)x.Key
    }).ToList();

该GroupBy已截断从时间DateTime了,所以你不必再调用它.

要使用DbFunctions.TruncateTime你需要引用程序集System.Data.Entity并包含using System.Data.Entity;

注意:编辑以解决弃用问题EntityFunctions.

Answer 2

尝试这个：

 .GroupBy(x => new {Year = x.LastLogIn.Year, Month = x.LastLogIn.Month, Day = x.LastLogIn.Day)
                .Select(x => new
                {
                    Value = x.Count(),
                    Year = x.Key.Year,
                    Month = x.Key.Month,
                    Day = x.Key.Day
                })

Answer 3

您可以轻松做到：

yourDateList.GroupBy(i => i.ToString("yyyyMMdd"))
             .Select(i => new
             {
                 Date = DateTime.ParseExact(i.Key, "yyyyMMdd", CultureInfo.InvariantCulture, DateTimeStyles.None),
                 Count = i.Count()
             });