C# 使用Lambda删除重复项

我需要一些帮助来使用lambda表达式删除实体框架上下文中的重复条目。我有一个包含以下列的表：

Id、事件日期、描述、员工Id、IncidentTypeId和IsAttendenceIncident

我想删除重复的条目，其中

DateOfIncident、EmployeeID、incidentypeid和isattendencident是相同的。

我想保留一个条目。我知道如何使用SQL中带有CTE的存储过程来完成此任务，但我不知道如何使用Lambda表达式来完成此任务

这段代码返回一个不包含重复项的列表，但现在如何删除不在此列表中的重复项

  var query = db.Incidents.Where(x => x.IsAttendanceIncident == "Y").GroupBy(x => new { x.EmployeeId, x.DateOfIncident, x.IsAttendanceIncident })
         .Select(x => x.FirstOrDefault());

更新：

所以我继续写了自定义的IEqualityComparer。现在，我如何使用id删除上下文中不属于我的特殊项的事件

 static void Main(string[] args)
    {
        DALIncidents.AttendanceEntities1 db = new DALIncidents.AttendanceEntities1();

       IEnumerable<DALIncidents.Incident> distinctItems = db.Incidents.Where(c => c.IsAttendanceIncident == "Y");
       distinctItems = distinctItems.Distinct(new DALIncidents.DistinctIncidentComparer());

       Console.ReadLine(); 
    }

static void Main（字符串[]args）
{
DALIncidents.AttendanceEntities1 db=新的DALIncidents.AttendanceEntities1（）；
IEnumerable differentitems=db.incents.Where（c=>c.IsAttendanceIncident==“Y”）；
differentitems=differentitems.Distinct（新的DALIncidents.differentincidentcomparer（））；
Console.ReadLine（）；
}

您需要使用Distinct函数，如果您只想使用一些字段，则需要创建一个相等比较器。（IEqualityComparer）

啊，刚刚看到上面的评论，请查看以下内容：

query2现在只包含重复项

var query = db.Incidents.Where(x => x.IsAttendanceIncident == "Y").GroupBy(x => new { x.EmployeeId, x.DateOfIncident, x.IsAttendanceIncident })
     .SelectMany(x => x.Skip(1));

例1：

    .Select(x => x.FirstOrDefault());  // your original code which retrieves entities to not delete

var dupes = db.Incidents.Except( query ); // get entities to delete

例2：

    .SelectMany( x => x.OrderBy( y => y.Id ).Skip(1) ); // gets dupes directly

var dupes = query; // already have what we need

最后：

foreach( var dupe in dupes )
{
    db.Incidents.Remove( dupe );
}

从我前面使用的测试上下文生成的示例SQL，其中Person实体与watches具有1:N关系：

C#：

生成的SQL:

SELECT 
1 AS [C1], 
[Skip1].[Id] AS [Id], 
[Skip1].[Brand] AS [Brand], 
[Skip1].[Person_Id] AS [Person_Id]
FROM  [dbo].[Persons] AS [Extent1]
CROSS APPLY  (SELECT [Project1].[Id] AS [Id], [Project1].[Brand] AS [Brand], [Project1].[Person_Id] AS [Person_Id]
    FROM ( SELECT [Project1].[Id] AS [Id], [Project1].[Brand] AS [Brand], [Project1].[Person_Id] AS [Person_Id], row_number() OVER (ORDER BY [Project1].[Id] ASC) AS [row_number]
        FROM ( SELECT 
            [Extent2].[Id] AS [Id], 
            [Extent2].[Brand] AS [Brand], 
            [Extent2].[Person_Id] AS [Person_Id]
            FROM [dbo].[Watches] AS [Extent2]
            WHERE [Extent1].[Id] = [Extent2].[Person_Id]
        )  AS [Project1]
    )  AS [Project1]
    WHERE [Project1].[row_number] > 1 ) AS [Skip1]

---

您是否检查过是否可以使用Distinct（）然后删除所有不是Distinct结果的内容（基于键）。Richard。我更新了问题。你能再看一眼吗。感谢您对IEqualityComparer的建议。这能正确地翻译成SQL吗？如果不需要，您只需指定

Where（group=>group.Count（）>1）

，然后在linq to objects中执行

Skip

；使用EF并不是解决问题的最佳方案，但您询问了如何使用EF完成此任务。EF不是最优的，因为您需要检索所有数据并构造实体对象，以便通过EF发出删除命令。对不起，我以为您是原始海报。在本地创建并枚举实体对象后，将执行跳过。我已经编辑了我之前的评论，其中描述了为什么它不是最佳执行解决方案-检索所有数据，必须创建实体对象才能在数据库中删除它们。然而，最初的海报要求EF解决方案，而不是最佳性能的解决方案（即存储过程），您是正确的；skip只能用于有序查询，在这种情况下，您只需更改SelectMany lambda表达式即可对iGroup集合的项进行排序，这是可行的。我删除了这个示例，因为它只是混淆了问题。在（#2）中添加了这个示例，它只检索要删除的项目。为以下代码生成的示例SQL，其中一个人与watchs:not fit:P有1:N关系

foreach( var dupe in dupes )
{
    db.Incidents.Remove( dupe );
}

context.Persons.SelectMany(x => x.Watches.OrderBy(y => y.Id).Skip(1))

SELECT 
1 AS [C1], 
[Skip1].[Id] AS [Id], 
[Skip1].[Brand] AS [Brand], 
[Skip1].[Person_Id] AS [Person_Id]
FROM  [dbo].[Persons] AS [Extent1]
CROSS APPLY  (SELECT [Project1].[Id] AS [Id], [Project1].[Brand] AS [Brand], [Project1].[Person_Id] AS [Person_Id]
    FROM ( SELECT [Project1].[Id] AS [Id], [Project1].[Brand] AS [Brand], [Project1].[Person_Id] AS [Person_Id], row_number() OVER (ORDER BY [Project1].[Id] ASC) AS [row_number]
        FROM ( SELECT 
            [Extent2].[Id] AS [Id], 
            [Extent2].[Brand] AS [Brand], 
            [Extent2].[Person_Id] AS [Person_Id]
            FROM [dbo].[Watches] AS [Extent2]
            WHERE [Extent1].[Id] = [Extent2].[Person_Id]
        )  AS [Project1]
    )  AS [Project1]
    WHERE [Project1].[row_number] > 1 ) AS [Skip1]