C# 为什么实体框架linq查询的内部SQL中没有GroupBy子句?
在实体框架的文档中: 在关于GroupBy的部分中,我们可以阅读以下代码:C# 为什么实体框架linq查询的内部SQL中没有GroupBy子句?,c#,sql,entity-framework,group-by,entity-framework-6,C#,Sql,Entity Framework,Group By,Entity Framework 6,在实体框架的文档中: 在关于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 (
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,我们就不能使用带OrderBy和不带连接的simpleselect吗?有人能解释一下上面的查询吗?底线是:因为SQL不能返回嵌套的结果集 每个SQL
SELECT
语句都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,GROUPBY
语句只返回分组列和聚合结果:
选择标准ID,计数(*)
来自学生
按标准ID分组;
剩下的学生专栏都不见了
LINQGroupBy
语句返回如下内容
standarid
StudentId StudentName
1.
21“学生1”
15“学生2”
2.
48“学生3”
91“学生4”
17“学生5”
因此,SQLGroupBy
语句永远不能作为LINQGroupBy
的源
实体框架(6)知道这一点,它生成一个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。但是为什么我们不能在没有连接和OrderBy的情况下进行简单的选择呢?因为“我们”什么都不做:)。EF决定创建这种查询,我认为是因为它保证了可以在客户端创建组。