使用Group By和Max將SQL轉換爲LINQ

Question

甚至可以使用LINQ編寫以下查詢嗎？我整天都在戰鬥，:(

select * from ProductGroup pg 
Inner join (
    select max(DateShipped) as ShipDate 
     , ProductId 
    from Lot lt 
    group by ProductId) lte 
    on pg.ProductId = lte.ProductId 

Answer 1

解決方法1

我使用LinqToSQL測試這對LinqPad

var data = from lt in Lots    
      group lt by lt.ProductId into grp 
      join pg in ProductGroups on grp.Key equals pg.ProductId 
      select new { pg.ProductId,pg.ProductName,ShipDate =grp.Max (g => g.ShipDate)}; 

data.Dump(); 

和它產生了這個SQL（幾乎和你的sql一樣，只是切換表的順序） ）

SELECT [t2].[ProductId], [t2].[ProductName], [t1].[value] AS [ShipDate] 
FROM (
    SELECT MAX([t0].[ShipDate]) AS [value], [t0].[ProductId] 
    FROM [Lot] AS [t0] 
    GROUP BY [t0].[ProductId] 
    ) AS [t1] 
INNER JOIN [ProductGroup] AS [t2] ON [t1].[ProductId] = [t2].[ProductId] 

溶液2

這是迫使相關子查詢，只要你想產生相同的結果，但沒有加入/組由

var data = from pg in ProductGroups 
      let sd = Lots.Where (l => l.ProductId == pg.ProductId).Max(l => l.ShipDate) 
      select new { pg.ProductId,pg.ProductName,ShipDate =sd}; 

    data.Dump(); 

，它產生的一種方式此SQL

SELECT [t0].[ProductId], [t0].[ProductName], (
    SELECT MAX([t1].[ShipDate]) 
    FROM [Lot] AS [t1] 
    WHERE [t1].[ProductId] = [t0].[ProductId] 
    ) AS [sd] 
FROM [ProductGroup] AS [t0] 

我希望它能幫助你

Answer 2

試試這個：

var result = (from pg in ProductGroup 
       join lte in 
       (
        from lt in Lot 
        group lt by lt.ProductId into gr 
        select new 
        { 
         ProductId = gr.Key, 
         ShipDate = gr.Max(x => x.DateShipped) 
        } 
       ) 
       on pg.ProductId equals lte.ProductId 
       select new 
       { 
        ID = pg.ProductId, 
        MaxShipDate = lte.ShipDate 
       }).ToList();