C# Linq到SQL：选择最新的不同条目

给定一个列为：[AuditEntityId][UserName][CaseID]的表

我想要一个具有最高[AuditEntityId]的特定[UserName]的[CaseID]的独特列表

基本上，我想要用户处理的最后五个案例，按从最新到最旧的顺序排列

我通过在[CaseID]上分组来实现distinct：

var lastItems = baseController.db.AuditEntities
                                 .OrderByDescending(a => a.AuditEntityId)
                                 .GroupBy(a => a.CaseID)
                                 .Select(a => a.FirstOrDefault())
                                 .Where(a => a.CaseID != null && a.CaseID != 0)
                                 .Where(a => a.UserName == filterContext.HttpContext.User.Identity.Name)
                                 .Take(5)
                                 .ToList();

这样做的目的是为我提供一个用户处理过的案例的清晰列表，但是完全忽略了

.OrderByDescending

。top linq语句转换为以下SQL：

SELECT TOP (5)
    [Limit1].[AuditEntityId] AS [AuditEntityId],
    [Limit1].[Reference] AS [Reference],
    [Limit1].[Timestamp] AS [Timestamp],
    [Limit1].[EntityName] AS [EntityName],
    [Limit1].[UserName] AS [UserName],
    [Limit1].[Action] AS [Action],
    [Limit1].[ComplaintId] AS [ComplaintId],
    [Limit1].[CaseID] AS [CaseID],
    [Limit1].[AuditReferencingStart] AS [AuditReferencingStart],
    [Limit1].[AuditReferencingEnd] AS [AuditReferencingEnd]
FROM
    (SELECT DISTINCT
        [Extent1].[CaseID] AS [CaseID]
    FROM
        [dbo].[AuditEntity] AS [Extent1] ) AS [Distinct1]
    CROSS APPLY (SELECT TOP (1)
        [Extent2].[AuditEntityId] AS [AuditEntityId],
        [Extent2].[Reference] AS [Reference],
        [Extent2].[Timestamp] AS [Timestamp],
        [Extent2].[EntityName] AS [EntityName],
        [Extent2].[UserName] AS [UserName],
        [Extent2].[Action] AS [Action],
        [Extent2].[ComplaintId] AS [ComplaintId],
        [Extent2].[CaseID] AS [CaseID],
        [Extent2].[AuditReferencingStart] AS [AuditReferencingStart],
        [Extent2].[AuditReferencingEnd] AS [AuditReferencingEnd]
    FROM
        [dbo].[AuditEntity] AS [Extent2]
    WHERE
        ([Distinct1].[CaseID] = [Extent2].[CaseID]) OR (([Distinct1].[CaseID] IS NULL) AND ([Extent2].[CaseID] IS NULL)) ) AS [Limit1]
WHERE
    ([Limit1].[CaseID] IS NOT NULL) AND ( NOT ((0 = [Limit1].[CaseID]) AND ([Limit1].[CaseID] IS NOT NULL))) AND (([Limit1].[UserName] = @p__linq__0))

提供的SQL中根本没有顺序。我可以将

.OrderByDescending

移动到

.GroupBy（a=>a.CaseID）之后。选择（a=>a.FirstOrDefault（））

，但它会在

顶部（1）

被

选择后对结果排序，这不会给我最新的审核条目

我还尝试使用的
.DistinctBy
，但使用此
.OrderByDescending
仍然无法按预期工作：

var lastItems = baseController.db.AuditEntities
                                 .Where(a => a.CaseID != null && a.CaseID != 0 && a.UserName == filterContext.HttpContext.User.Identity.Name)
                                 .DistinctBy(a => a.CaseID)
                                 .OrderBy(a => a.AuditEntityId)
                                 .Take(5)
                                 .ToList();

您需要对结果集进行排序。试一试

var lastItems = baseController.db.AuditEntities                                 
                             .GroupBy(a => a.CaseID)
                             .Select(a => a.FirstOrDefault())
                             .Where(a => a.CaseID != null && a.CaseID != 0)
                             .Where(a => a.UserName == filterContext.HttpContext.User.Identity.Name)
                             .OrderByDescending(a => a.AuditEntityId)
                             .Take(5)
                             .ToList();

当您在按审核实体ID排序后按案例ID进行分组，然后执行其他操作时，该排序不会对结果集产生影响

编辑

如果不知道确切的模式，我无法确定。但是通过“我想要一个具有最高[AuditEntityId]的特定[UserName]的[CaseID]的不同列表”，您可以尝试以下方法

.db.AuditEntities
    .Where(a => a.CaseID != null 
                && a.CaseID != 0
                && a.UserName == filterContext.HttpContext.User.Identity.Name)
    .GroupBy(a => a.CaseID)
    .OrderByDescending(grp => grp.Max(g => g.AuditEntityId))        
    .Take(5)
    .Select(a => a.FirstOrDefault())
    .ToList();

您需要对结果集进行排序。试一试

var lastItems = baseController.db.AuditEntities                                 
                             .GroupBy(a => a.CaseID)
                             .Select(a => a.FirstOrDefault())
                             .Where(a => a.CaseID != null && a.CaseID != 0)
                             .Where(a => a.UserName == filterContext.HttpContext.User.Identity.Name)
                             .OrderByDescending(a => a.AuditEntityId)
                             .Take(5)
                             .ToList();

当您在按审核实体ID排序后按案例ID进行分组，然后执行其他操作时，该排序不会对结果集产生影响

编辑

如果不知道确切的模式，我无法确定。但是通过“我想要一个具有最高[AuditEntityId]的特定[UserName]的[CaseID]的不同列表”，您可以尝试以下方法

.db.AuditEntities
    .Where(a => a.CaseID != null 
                && a.CaseID != 0
                && a.UserName == filterContext.HttpContext.User.Identity.Name)
    .GroupBy(a => a.CaseID)
    .OrderByDescending(grp => grp.Max(g => g.AuditEntityId))        
    .Take(5)
    .Select(a => a.FirstOrDefault())
    .ToList();

但是，如果在分组后I
OrderBy
，它是在
之后排序的。Select（a=>a.FirstOrDefault（））
这意味着最新条目不是“first”条目selected如果在分组后I
OrderBy
，它是在
之后排序的。Select（a=>a.FirstOrDefault（））
这意味着最近的条目不是“第一个”选择的条目