Question

这是Linq for Left Join -

var Records = from cats in Context.Categories
         join prods in Context.Products on cats.Id equals prods.Category_Id into Cps
         from results in Cps.DefaultIfEmpty()
         select new { CatName = cats.Name, ProdName = results.Name };

这是Lambda Expression for same -

var Records = Context.Categories.GroupJoin(Context.Products, c => c.Id, p => p.Category_Id, (c, p) => new { CatName = c.Name, Prods = p });

Linq使用SQL -

SELECT 
[Extent1].[Id] AS [Id], 
[Extent1].[Name] AS [Name], 
[Extent2].[Name] AS [Name1]
FROM  [dbo].[Categories] AS [Extent1]
LEFT OUTER JOIN [dbo].[Products] AS [Extent2] ON [Extent1].[Id] = [Extent2].[Category_Id]

Lambda使用以下SQL -

 SELECT 
 [Project1].[Id] AS [Id], 
 [Project1].[Name] AS [Name], 
 [Project1].[C1] AS [C1], 
 [Project1].[Category_Id] AS [Category_Id], 
 [Project1].[Description] AS [Description], 
 [Project1].[Id1] AS [Id1], 
 [Project1].[Name1] AS [Name1], 
 [Project1].[Price] AS [Price]
 FROM ( SELECT 
[Extent1].[Id] AS [Id], 
[Extent1].[Name] AS [Name], 
[Extent2].[Category_Id] AS [Category_Id], 
[Extent2].[Description] AS [Description], 
[Extent2].[Id] AS [Id1], 
[Extent2].[Name] AS [Name1], 
[Extent2].[Price] AS [Price], 
CASE WHEN ([Extent2].[Category_Id] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
FROM  [dbo].[Categories] AS [Extent1]
LEFT OUTER JOIN [dbo].[Products] AS [Extent2] ON [Extent1].[Id] = [Extent2].[Category_Id])  AS [Project1]
ORDER BY [Project1].[Id] ASC, [Project1].[C1] ASC

我的问题是为什么同一个语句返回不同的SQL？

Answer 1

根本不是同一个查询 - 你确实在lambda表达式版本中有GroupJoin - 但是你没有得到与SelectMany相对应的：{/ p>

from results in Cps.DefaultIfEmpty()

由于引入了透明标识符，确切的查询转换变得有点复杂，但我确信这是不同的。

Answer 2

除了Jon的回复，你还有：

var Records = Context.Categories.GroupJoin(Context.Products, c => c.Id, p => p.Category_Id, (c, p) => new { CatName = c.Name, Prods = p });

其中Prods = p（拉动整个产品记录）Linq查询具有ProdName = results.Name（仅从分组结果中提取名称字符串）。这是相当多的差异。

尝试将Linq输入LinqPad并查看结果窗格中生成的Lamba的内容。

由同一Linq和Lambda表达式生成的两个不同的SQL语句

2 个答案: