LINQ 左连接和 GroupBy

Question

假设我有这个 SQL 查询：

SELECT 
    PromotionSlot.StaffFName, 
    SUM(PromotionSlot.Max_Occupancy) AS TOTAL,
    SUM(DISTINCT(Booking.Quantity)) AS Occupied
From PromotionSlot 
LEFT JOIN Booking
ON PromotionSlot.StaffID=Booking.StaffID
GROUP BY  PromotionSlot.StaffFName

结果：

|StaffFName    |TOTAL    |Occupied|
-----------------------------------
|Jason         |13       |1       |
|John Doe      |9        |0       |      
|Marry Jane    |7        |2       |

这是我的数据库Table:

PromotionSlot TABLE: ID(PK),Max_Occupancy,StaffFName..., StaffID(FK)
Booking TABLE: ID(PK), Quantity...,StaffID(FK)

如何将其转换为 LINQ？这是我的尝试：

var staffData = 
    (from ps in dc.PromotionSlots
     join b in dc.Bookings on ps.StaffID = b.StaffID
     group ps by ps.StaffFName into NewGroup
     select new dataView
     {
         StaffFName = NewGroup.Key,
         Total = NewGroup.Sum(a => a.Max_Occupancy),

         //problem:
         //Occupied = NewGroup.Sum(b => b.Quantity)
     }

计划有 Occupied = NewGroup.Sum(b => b.Quantity) 但是当我尝试将 b 指向 Booking table 的 quantity 列时它显示错误（红色-行），我认为问题来自 group ps by ps.StaffFName into NewGroup 使其可用于 PromotionSlot table 而不是 Booking table。但是我完全不知道如何解决这个问题！

Answer 1

根据您的 SQL 查询，您需要的是 Distinct 个数量和 Sum 个数量。

var staffData = 
    (from ps in dc.PromotionSlots
     join b in dc.Bookings on ps.StaffID = b.StaffID into slots
     from slot in slots.DefaultIfEmpty()
     group new {ps, slot} by ps.StaffFName into NewGroup
     select new dataView
     {
         StaffFName = NewGroup.Key,
         Total = NewGroup.Sum(a => a.ps!=null? a.ps.Max_Occupancy: 0),

         //problem:
         Occupied = NewGroup.Select(x=>x.slot.Quantity).Distinct().Sum()
     }

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