Question

我有两张表Task和EmpTask。

Table: Task    Table: EmpTask
__________     ________________________
ID | Title     ID | TaskID  | XXXX
__________     ________________________
1  | task1     1  | 1       | emptask1
2  | task2     2  | 1       | emptask2  (this is last occurance of taskID 1)
__________     3  | 2       | emptask3    
               4  | 2       | emptask4  (this is last occurance of taskID 2)
               ________________________

预期结果：

________________________
ID | Title   | XXXX
________________________
1  | task1   | emptask2
2  | task2   | emptask4
________________________

我想要帮助在Lambda表达式中编写查询。

提前致谢。

Answer 1

假设您想要max EmpTask。

var result = (from t in db.Task
         join r in db.EmpTask
                      .GroupBy(i => i.TaskID)
                      .Select(i => new { TaskID = i.Key, EmpTaskId = i.Max(t => t.ID)})
                  on t.ID equals r.TaskID
         join et in db.EmpTask on r.EmpTaskId equals et.ID
         select new
         {
            ID = t.ID,
            Title = t.Title,
            XXXX = et.XXXX
         }).ToList();

Lambda Expression

var result = db.Task.Join(
            db.EmpTask
                      .GroupBy(i => i.TaskID)
                      .Select(i => new { TaskID = i.Key, EmpTaskId = i.Max(t => t.ID)})
                  ,
            t => t.ID,
            r => r.TaskID,
            (t, r) => new { Task = t, EmpTaskTmp = r}
         ).Join(
            db.EmpTask,
            et => et.ID
            y => y.EmpTaskTmp.EmpTaskId,
            (et, y) => new { Task = y.Task, MaxEmpTask = et}
         ).Select(i => new
         {
            ID = i.Task.ID,
            Title = i.Task.Title,
            XXXX = i.MaxEmpTask.XXXX
         });

Answer 2

我管理得到答案。谢谢大家的帮助:)

var result = db.Task.Join(db.EmpTask, t => t.ID, et => et.TaskID, (t, et) => new { t, et })
                    .Select(m => new  
                    {
                        ID = m.t.ID,
                        Title = m.t.Title,
                        XXXX= m.et.XXXX
                    }).GroupBy(m => m.ID, (key, g) => g.OrderByDescending(m => m.EmpTaskId).FirstOrDefault()).ToList();

Lambda表达式与最后一次出现的右表联接

2 个答案: