[英]LINQ returning records with most recent dates with a join
我正在嘗試創建LINQ語句,該語句返回具有最近分配的任務歷史記錄日期的記錄(任務)列表。 因此,有兩個表,第一個是任務表,其中包含有關單個任務的所有信息,然后是任務歷史表,用於跟蹤任務分配的方式和對象。 這兩個表都具有TaskID,可以在聯接時鏈接它們。 問題是在任務歷史記錄中可以有許多具有相同TaskID的記錄,但我只想返回具有最近分配日期的記錄,該記錄是任務歷史記錄表中的一列。 我怎樣才能做到這一點?
這是我到目前為止的內容:
var query = from task in db.AP_Tasks
join taskH in db.AP_TaskHistory on task.TaskID equals taskH.TaskID
orderby new {taskH.AssignDate}
select new {task = task, assignedDate = taskH.AssignDate};
更新:
最新代碼:
var query = from task in db.AP_Tasks
from th in db.AP_TaskHistory
where th.TaskID == task.TaskID
orderby th.AssignDate
select new {task = task, assignDate = th.AssignDate};
您需要的是CROSS APPLY樣式查詢。
select AP_Tasks.*, history.AssignDate
from AP_Tasks
cross apply (
select top 1 AP_TaskHistory.AssignDate
from AP_TaskHistory
where AP_Tasks.TaskID = AP_TaskHistory.TaskID
order by AP_TaskHistory.AssignDate DESC
) history
請參閱此處: 如何在LINQ-to-SQL中編寫此交叉應用查詢?
我認為類似這樣的方法對您有用。
var query = (from task in db.AP_Tasks
join taskH in db.AP_TaskHistory on task.TaskID equals taskH.TaskID
orderby new {taskH.AssignDate} descending
select new {task = task, assignedDate = taskH.AssignDate}).First();
警告,我還沒有測試過。
除了.First()
您還可以嘗試.FirstOrDefault()
以便更輕松地處理異常。
這是使您前進的示例:
List<Task> tasks = new List<Task>();
tasks.Add(new Task {TaskID = 1});
tasks.Add(new Task {TaskID = 3});
tasks.Add(new Task {TaskID = 5});
List<TaskHistory> taskHistories = new List<TaskHistory>();
taskHistories.Add(new TaskHistory { TaskID = 3, AssignDate = DateTime.Now });
taskHistories.Add(new TaskHistory { TaskID = 3, AssignDate = DateTime.Now.AddDays(3) });
taskHistories.Add(new TaskHistory { TaskID = 1, AssignDate = DateTime.Now });
taskHistories.Add(new TaskHistory { TaskID = 1, AssignDate = DateTime.Now.AddDays(1) });
taskHistories.Add(new TaskHistory { TaskID = 1, AssignDate = DateTime.Now.AddDays(4) });
var query = from task in tasks
join thist in taskHistories on task.TaskID equals thist.TaskID
group new { t = task, date = thist.AssignDate } by task.TaskID into grouped
from g in grouped
where g.date == grouped.Max(taskInGroup => taskInGroup.date)
select g;
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