Lambda 表达式 group by select last 有一个条件答案

【问题标题】：Lambda expression group by select last with a conditionLambda 表达式 group by select last 有一个条件
【发布时间】：2017-03-07 23:09:21
【问题描述】：

使用此代码：

List<Transaction> list = db.Transaction
                           .GroupBy(t => t.ProcessId)
                           .Select(g => g.OrderByDescending(t => t.TransactionId).FirstOrDefault())
                           .Where(t => t.xId != null && t.PAmount > 0)
                           .ToList();

我试图获取xId 不为空的最后一条记录.. 分组工作正常.. 但即使xId 为空，它也会让我获得最后一条记录

任何帮助！

【问题讨论】：

标签： c# sql-server linq lambda

【解决方案1】：

分组前移动过滤：

db.Transaction
  .Where(t => t.xId != null && t.PAmount > 0)
  .GroupBy(t => t.ProcessId)
  .Select(g => g.OrderByDescending(t => t.TransactionId).FirstOrDefault())
  .ToList();

因此，您将获得每个进程的 id 和金额大于零的最新交易。否则，如果最新交易的 id 等于 null，您将完全过滤掉该进程交易。

EF 会生成类似的查询

SELECT
    [Limit1].[TransactionId] AS [TransactionId],
    [Limit1].[xId] AS [xId],
    [Limit1].[PAmount] AS [PAmount],
    [Limit1].[ProcessId] AS [ProcessId]
    FROM 
    (SELECT DISTINCT
        [Extent1].[ProcessId] AS [ProcessId]
        FROM [dbo].[Transactions] AS [Extent1]
        WHERE ([Extent1].[xId] IS NOT NULL) AND ([Extent1].[PAmount] > 0)
    ) AS [Distinct1]
    OUTER APPLY
    (SELECT TOP (1) 
       [Project2].[TransactionId] AS [TransactionId],
       [Project2].[xId] AS [xId],
       [Project2].[PAmount] AS [PAmount],
       [Project2].[ProcessId] AS [ProcessId]
       FROM ( SELECT
            [Extent2].[TransactionId] AS [TransactionId],
            [Extent2].[xId] AS [xId],
            [Extent2].[PAmount] AS [PAmount],
            [Extent2].[ProcessId] AS [ProcessId]
            FROM [dbo].[Transactions] AS [Extent2]
            WHERE ([Extent2].[xId] IS NOT NULL) AND ([Extent2].[PAmount] > 0)
                   AND ([Distinct1].[ProcessId] = [Extent2].[ProcessId])
            ) AS [Project2]
        ORDER BY [Project2].[TransactionId] DESC
    ) AS [Limit1]

您可以看到 xId NOT NULL 和 PAmount > 0 条件应用了两次 - 第一次获得不同的进程 ID（具有适当的事务）。第二次为每个进程选择最新事务。

【讨论】：

@AmrIbrahim 不起作用是什么意思？错误？
@AmrIbrahim 我添加了由 EF 生成的查询。您的情况不可能没有应用过滤条件