Linq to SQL：SELECT 个最新的 DISTINCT 条目

Question

给定 table 列：[AuditEntityId] [UserName] [CaseID]

我想要具有最高 [AuditEntityId] 的特定 [UserName] 的不同 [CaseID] 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 [CaseID] 分组来实现不同：

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();

这实现了给我一个用户处理过的案例的独特列表的目标，但是 .OrderByDescending 被完全忽略了。最上面的linq语句转化为下面的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))

所提供的 SQL 中根本没有 ORDER。我可以将 .OrderByDescending 移动到 .GroupBy(a => a.CaseID).Select(a => a.FirstOrDefault()) 之后，但是它会在 TOP (1) 被 SELECTed 之后排序结果，这不会给我最新的审计条目.

我也尝试使用 MoreLinq 的 .DistinctBy，但是 .OrderByDescending 仍然无法正常工作：

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();

Answer 1

您需要订购您的结果集。尝试

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();

当你在Order By AuditEntityId之后Group By CaseID，接着进行其他操作时，那个顺序OrderBy对结果集没有影响。

编辑

在不知道确切架构的情况下，我无法确定。但是通过"I want a distinct list of [CaseID]'s for a specific [UserName] that has the highest [AuditEntityId]"，你可以试试这个

.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();

Linq to SQL：SELECT 个最新的 DISTINCT 条目

Linq to SQL: SELECT latest DISTINCT entries

c#

sql-server

linq

linq-to-sql

distinct