Question

我一直在看以下帖子并试图将它应用到我的但没有运气： LINQ Inner-Join vs Left-Join

我有下面的查询，每次运行它都会返回0条记录：

        var tasks = from tt in d.luProjectTaskTypes
                    join cbt in d.CostByTasks
                      on tt.ProjectTaskTypeID equals cbt.ProjectTaskTypeID into temp
                    from cbt in temp.DefaultIfEmpty()
                    where cbt.ProposalID == Convert.ToInt32(this.StateItems["PropNumber"])  || cbt.ProposalID == null
                    select new
                    {
                        ProposalId = (cbt.ProposalID == null ? Convert.ToInt32(this.StateItems["PropNumber"]) : cbt.ProposalID),
                        TaskId = tt.ProjectTaskTypeID,
                        CostByTaskId = (cbt.CostByTaskID == null ? 0 : cbt.CostByTaskID),
                        TypeOfWork = tt.ProjectTaskType,
                        AmountRequested = (cbt.AmountRequested == null ? 0 : cbt.AmountRequested),
                        CostShare = (cbt.CostShareAmount == null ? 0 : cbt.CostShareAmount)
                    };

其中luProjectTaskTypes是包含选项列表的查找表。我希望为此表中的每个条目返回一条记录，无论它是否在CostByTasks中匹配，但我总是得到0.我做错了什么？！

更新

这是它正在生成的SQL -

SELECT 
    (CASE 
        WHEN ([t1].[ProposalID]) IS NULL THEN @p1
        ELSE [t1].[ProposalID]
     END) AS [ProposalId], [t0].[ProjectTaskTypeID] AS [TaskId], 
    (CASE 
        WHEN ([t1].[CostByTaskID]) IS NULL THEN @p2
        ELSE [t1].[CostByTaskID]
     END) AS [CostByTaskId], [t0].[ProjectTaskType] AS [TypeOfWork], 
    (CASE 
        WHEN [t1].[AmountRequested] IS NULL THEN CONVERT(Decimal(33,4),@p3)
       ELSE CONVERT(Decimal(33,4),[t1].[AmountRequested])
     END) AS [AmountRequested], 
    (CASE 
        WHEN [t1].[CostShareAmount] IS NULL THEN CONVERT(Decimal(33,4),@p4)
        ELSE CONVERT(Decimal(33,4),[t1].[CostShareAmount])
     END) AS [CostShare]
FROM [frgprop].[luProjectTaskType] AS [t0]
LEFT OUTER JOIN [frgprop].[CostByTask] AS [t1] ON [t0].[ProjectTaskTypeID] = [t1].[ProjectTaskTypeID]
WHERE ([t1].[ProposalID] = @p0) OR (([t1].[ProposalID]) IS NULL)

Answer 1

您的问题出在WHERE子句

中

当你LEFT JOIN一个表时，没关系，但是如果你在LEFT-JOINED表上设置标准，它基本上把它变成一个INNER JOIN。你应该允许Null通过它。

where cbt.ProposalID == Convert.ToInt32(this.StateItems["PropNumber"]) OR cbt.ProposalID is NULL

我不确定is NULL语法怎么样 - 可能必须是db.null(field)等等 - 你必须检查一下;但这个概念是有效的。

Answer 2

不确定这是否有帮助，但您可以尝试将where子句移动到连接中 -

var tasks = from tt in d.luProjectTaskTypes
            join cbt in d.CostByTasks
              on new {ptid = tt.ProjectTaskTypeID,  pid = cbt.ProposalID } equals new { ptid = cbt.ProjectTaskTypeID, pid = Convert.ToInt32(this.StateItems["PropNumber"] } into temp
            from cbt in temp.DefaultIfEmpty()
            select new
            {
                ProposalId = (cbt.ProposalID == null ? Convert.ToInt32(this.StateItems["PropNumber"]) : cbt.ProposalID),
                TaskId = tt.ProjectTaskTypeID,
                CostByTaskId = (cbt.CostByTaskID == null ? 0 : cbt.CostByTaskID),
                TypeOfWork = tt.ProjectTaskType,
                AmountRequested = (cbt.AmountRequested == null ? 0 : cbt.AmountRequested),
                CostShare = (cbt.CostShareAmount == null ? 0 : cbt.CostShareAmount)
            };

你需要担心检查cbt.ProposalID == null而它应该生成这个sql -

 ...   FROM [frgprop].[luProjectTaskType] AS [t0]
    LEFT OUTER JOIN [frgprop].[CostByTask] AS [t1] ON [t0].[ProjectTaskTypeID] = [t1].[ProjectTaskTypeID] AND [t1].[ProposalID] = @p0

<强>更新以下是编译的更新版本。一些小的改变让它发挥作用。

var tasks = from tt in d.luProjectTaskTypes
                        join cbt in d.CostByTasks
                          on new {tt.ProjectTaskTypeID, ProposalID = Convert.ToInt32(this.StateItems["PropNumber"]) } equals new {cbt.ProjectTaskTypeID, cbt.ProposalID} into temp
                        from cbt in temp.DefaultIfEmpty()
                        select new
                        {
                            ProposalId = (cbt.ProposalID == null ? Convert.ToInt32(this.StateItems["PropNumber"]) : cbt.ProposalID),
                            TaskId = tt.ProjectTaskTypeID,
                            CostByTaskId = (cbt.CostByTaskID == null ? 0 : cbt.CostByTaskID),
                            TypeOfWork = tt.ProjectTaskType,
                            AmountRequested = (cbt.AmountRequested == null ? 0 : cbt.AmountRequested),
                            CostShare = (cbt.CostShareAmount == null ? 0 : cbt.CostShareAmount)
                        };

无法获得Left JOIN linq查询工作！

2 个答案: