为什么在 Entity Framework linq 查询的内部 SQL 中没有 GroupBy 子句？

Question

在 Entity Framework 的文档中： https://www.entityframeworktutorial.net/querying-entity-graph-in-entity-framework.aspx

在有关 GroupBy 的部分中，我们可以阅读以下代码：

using (var ctx = new SchoolDBEntities())
{    
    var students = from s in ctx.Students 
                group s by s.StandardId into studentsByStandard
                select studentsByStandard;

    foreach (var groupItem in students)
    {
        Console.WriteLine(groupItem.Key);

        foreach (var stud in groupItem)
        {
            Console.WriteLine(stud.StudentId);
        }
    }
}

在内部执行 SQL:

SELECT 
[Project2].[C1] AS [C1], 
[Project2].[StandardId] AS [StandardId], 
[Project2].[C2] AS [C2], 
[Project2].[StudentID] AS [StudentID], 
[Project2].[StudentName] AS [StudentName], 
[Project2].[StandardId1] AS [StandardId1]
FROM ( SELECT 
    [Distinct1].[StandardId] AS [StandardId], 
    1 AS [C1], 
    [Extent2].[StudentID] AS [StudentID], 
    [Extent2].[StudentName] AS [StudentName], 
    [Extent2].[StandardId] AS [StandardId1], 
    CASE WHEN ([Extent2].[StudentID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C2]
    FROM   (SELECT DISTINCT 
        [Extent1].[StandardId] AS [StandardId]
        FROM [dbo].[Student] AS [Extent1] ) AS [Distinct1]
    LEFT OUTER JOIN [dbo].[Student] AS [Extent2] ON ([Distinct1].[StandardId] = [Extent2].                                                [StandardId]) OR (([Distinct1].[StandardId] IS NULL) AND ([Extent2].[StandardId] IS NULL))
)  AS [Project2]
ORDER BY [Project2].[StandardId] ASC, [Project2].[C2] ASC
go

为什么SQL中没有GroupBy子句？如果不需要 GroupBy 子句，我们不能只使用带有 OrderBy 而没有 Joins 的简单 Select 吗？谁能解释一下上面的查询？

Answer 1

底线是：因为SQL不能return嵌套结果集。

每个 SQL SELECT 语句 return 都是一个简单的值列表。 LINQ 能够 returning 对象图，即具有嵌套对象的对象。这正是 LINQ 的 GroupBy 所做的。

在 SQL 中，一个 GROUP BY 语句仅 return 分组列和汇总结果：

SELECT StandardId, COUNT(*)
FROM Students
GROUP BY StandardId;

其余的学生专栏都没有了。

LINQ GroupBy 语句 return 类似于

StandardId
            StudentId StudentName
1
            21        "Student1"
            15        "Student2"
2
            48        "Student3"
            91        "Student4"
            17        "Student5"

因此，SQL GROUP BY 语句永远不能成为 LINQ GroupBy.

的源

Entity Framework (6) 知道这一点并生成一个 SQL 语句，从数据库中提取所有必需的数据，添加一些使分组更容易的部分，并在客户端创建分组.

为什么在 Entity Framework linq 查询的内部 SQL 中没有 GroupBy 子句？

Why there is no GroupBy clause in internal SQL of Entity Framework linq query?

c#

sql

entity-framework

group-by

entity-framework-6