c# - 使用 Lambda 删除重复项

Question

我需要一些帮助来使用 lambda 表达式来删除我的 Entity Framework 上下文中的重复条目。我有一个包含以下列的表格：

Id, DateOfIncident, Description, EmployeeId, IncidentTypeId, and IsAttendanceIncident

我想删除DateOfIncident, EmployeeID, IncidentTypeID and IsAttendanceIncident are the same. 我确实想保留一个条目的重复条目。我知道如何在 SQL 中使用带有 CTE 的存储过程来执行此操作，但我无法弄清楚如何使用 Lambda 表达式来完成此任务。

此代码返回一个列表，不包括我的重复项，但现在我该如何删除那些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：

所以我继续编写了自定义的 IEqualityComparer。现在如何使用 id 删除我的上下文中不在我 distinctItems 中的事件？

 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(); 
    }

score 9 · Accepted Answer

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

示例 1：

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

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

示例 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 );
}

从我之前使用的测试上下文生成的示例 SQL，其中 Person 实体与手表具有 1:N 关系：

C＃：

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

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]

score 1 · Accepted Answer

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


var query2 = from duplicate in db.Incidents
                 .Where(x => x.IsAttendanceIncident == "Y" && !query.Any(i => i.Id == duplicate.Id));

query2 现在将只包含重复项？

score 0 · Accepted Answer

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

score 0 · Accepted Answer

您将需要使用 Distinct 功能，如果您只想使用某些字段，则需要创建一个平等比较器。(IEqualityComparer)

啊刚刚看到上面的评论，查看更多：

使用 linq 删除列表中的重复项

c# - 使用 Lambda 删除重复项

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