Question

我正在将我的项目转换为使用EF，并且还希望将存储过程转换为Linq-to-entities查询。

我的SQL查询（简单版本），我无法转换：

SELECT 
       CategoryID, Title as CategoryTitle,Description, 
       LastProductTitle,LastProductAddedDate
FROM
(
    SELECT
        C.CategoryID, C.Title,C.Description, C.Section,
        P.Title as LastProductTitle, P.AddedDate as LastProductAddedDate,                                          
        ROW_NUMBER() OVER (PARTITION BY P.CategoryID ORDER BY P.AddedDate DESC) AS Num

    FROM 
         Categories C         
         LEFT JOIN Products P ON P.CategoryID = C.CategoryID
) OuterSelect

WHERE 
     OuterSelect.Num = 1

单词：我想返回所有类别（来自Categories表）以及最后添加到此类别的产品（从Products表）添加的标题和日期。

如何使用实体框架工作查询实现此目的？以最有效的方式。

Answer 1

在这种情况下是否需要Row_Number（）？

以下内容返回Northwind数据库中所有客户的最新订单：

from c in Customers
join o in Orders on c.CustomerID equals o.CustomerID
group o by  new { c.CustomerID, c.ContactName}  into g
select new {
 CustomerID = g.Key.CustomerID,
 Name = g.Key.ContactName,
 OrderDate = g.Max(a=> a.OrderDate)
}

创建的查询（来自LinqPad）是

SELECT MAX([t1].[OrderDate]) AS [OrderDate], [t0].[CustomerID], [t0].[ContactName] AS [Name]
FROM [Customers] AS [t0]
INNER JOIN [Orders] AS [t1] ON [t0].[CustomerID] = [t1].[CustomerID]
GROUP BY [t0].[CustomerID], [t0].[ContactName]

这个改为左连接

from c in Customers
         join o in Orders on c.CustomerID equals o.CustomerID into g
         from a in g.DefaultIfEmpty()
         group a by new { c.CustomerID, c.ContactName}  into g
         select new
         {
             g.Key.CustomerID,
             g.Key.ContactName,
             RecentOrder  = g.Max(a=> a.OrderDate) 
         }

，生成的查询是

SELECT MAX([t1].[OrderDate]) AS [RecentOrder], [t0].[CustomerID], [t0].[ContactName]
FROM [Customers] AS [t0]
LEFT OUTER JOIN [Orders] AS [t1] ON [t0].[CustomerID] = [t1].[CustomerID]
GROUP BY [t0].[CustomerID], [t0].[ContactName]

具有Linq到实体的SQL Row_number

1 个答案: