c# - 为什么 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);
        }
    }
}

内部执行以下 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，我们不能只使用简单的 Select with OrderBy 而没有 Joins 吗？谁能解释一下上面的查询？

Answer 1

底线是:因为 SQL 不能返回嵌套的结果集。

每个 SQL SELECT 语句都会返回一个简单的值列表。 LINQ 能够返回对象图，即具有嵌套对象的对象。这正是 LINQ 的 GroupBy 所做的。

在 SQL 中，GROUP BY 语句仅返回分组列和聚合结果:

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

其余的学生栏都不见了。

LINQ GroupBy 语句返回类似的内容

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，它会生成一个 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。