Question

我正在调查LINQ被映射到SQL的方式，我遇到了一些麻烦。

有一个表sessions，每个用户登录一行。请考虑以下SQL


SELECT COUNT(*) AS c
FROM sessions
GROUP BY sessions.user_id
ORDER BY c DESC

我正在使用LINQPad来测试LINQ-to-SQL转换。

海峡前进


    from s in Sessions group s by s.User_id into logins 
    orderby logins.Count() descending
    select new { c = logins.Count() }

给了我


SELECT [t1].[value2] AS [c]
FROM (
    SELECT COUNT(*) AS [value], COUNT(*) AS [value2]
    FROM [sessions] AS [t0]
    GROUP BY [t0].[user_id]
    ) AS [t1]
ORDER BY [t1].[value] DESC

修改后的


    (from s in Sessions group s by s.User_id into logins 
    select new { c = logins.Count() }).OrderByDescending(v => v.c)

被映射到


SELECT [t1].[value2] AS [c]
FROM (
    SELECT COUNT(*) AS [value], COUNT(*) AS [value2]
    FROM [sessions] AS [t0]
    GROUP BY [t0].[user_id]
    ) AS [t1]
ORDER BY [t1].[value] DESC

我无法获得这种最佳输出。有办法吗？
或者，也许，我不应该担心SQL会得到优化吗？

Answer 1

该查询存在“无”错误，如果您担心的话，它不会对行进行两次COUNT。

Answer 2

或者，也许，我不应该担心SQL会得到优化吗？

比较“最佳”sql和生成的sql的执行计划。（Sql Studio - 点击“显示估计执行计划”）

通过检查和绘制我多年来点击该按钮 - 生成的子查询根本没有任何问题。

即使这个查询也应该没问题（意思是：不需要额外的费用）

SELECT [t1].[value2] AS [c] 
FROM ( 
    SELECT COUNT(*) AS [value], COUNT(*) AS [value2] 
    FROM [sessions] AS [t0] 
    GROUP BY [t0].[user_id] 
    ) AS [t1] 
ORDER BY [t1].[value] DESC

不明确的LINQ to SQL映射

2 个答案: