改进LINQ生成的SQL查询到EF

Question

我有学生表,我想编写一个Linq查询来获取多个计数.Linq生成的SQL查询过于复杂且未经优化.

以下是我的表的定义:

[Id] [int] IDENTITY(1,1) NOT NULL,
[Name] [varchar](100) NULL,
[Age] [int] NULL,

我需要对名为= test的学生进行一次计数,为年龄> 10的学生计算一次.这是我尝试过的一个查询:

 var sql = from st in school.Students
                          group st by 1 into grp
                          select new
                          {
                              NameCount = grp.Count(k => k.Name == "Test"),
                              AgeCount = grp.Count(k => k.Age > 5)
                          };

生成的SQL查询是:

SELECT 
    [Limit1].[C1] AS [C1], 
    [Limit1].[C2] AS [C2], 
    [Limit1].[C3] AS [C3]
    FROM ( SELECT TOP (1) 
        [Project2].[C1] AS [C1], 
        [Project2].[C2] AS [C2], 
        (SELECT 
            COUNT(1) AS [A1]
            FROM [dbo].[Student] AS [Extent3]
            WHERE ([Project2].[C1] = 1) AND ([Extent3].[Age] > 5)) AS [C3]
        FROM ( SELECT 
            [Distinct1].[C1] AS [C1], 
            (SELECT 
                COUNT(1) AS [A1]
                FROM [dbo].[Student] AS [Extent2]
                WHERE ([Distinct1].[C1] = 1) AND (N'Test' = [Extent2].[Name])) AS [C2]
            FROM ( SELECT DISTINCT 
                1 AS [C1]
                FROM [dbo].[Student] AS [Extent1]
            )  AS [Distinct1]
        )  AS [Project2]
    )  AS [Limit1]

对我来说,这似乎很复杂.这可以通过以下简单查询来实现:

select COUNT(CASE WHEN st.Name = 'Test' THEN 1 ELSE 0 END) NameCount,
 COUNT(CASE WHEN st.Age > 5 THEN 1 ELSE 0 END) AgeCount from Student st

在LINQ中是否有一种方法可以使用生成的SQL查询同时具有聚合而不是将两个单独的查询与嵌套查询相结合？

Answer 1

根据我使用EF6的经验,条件Sum(ie Sum(condition ? 1 : 0))转换为SQL比Count使用谓词(即Count(condition))更好:

var query =
    from st in school.Students
    group st by 1 into grp
    select new
    {
        NameCount = grp.Sum(k => k.Name == "Test" ? 1 : 0),
        AgeCount = grp.Sum(k => k.Age > 5 ? 1 : 0)
    };

顺便说一句,您的SQL示例也应该使用SUM.为了利用COUNT排除NULLs 的SQL ,它应该是ELSE NULL或不是ELSE:

select COUNT(CASE WHEN st.Name = 'Test' THEN 1 END) NameCount,
    COUNT(CASE WHEN st.Age > 5 THEN 1 END) AgeCount
    from Student st

但是没有相同的LINQ构造,因此无法让EF6生成这样的转换.但IMO SUM足够相当.