这些LINQ查询之间有什么区别

Question

我一直在使用一些LINQ over Entities,我得到了奇怪的结果,我想得到解释......

给出以下LINQ查询,

// Sample # 1
IEnumerable<GroupInformation> groupingInfo;
groupingInfo = from a in context.AccountingTransaction
               group a by a.Type into grp
               select new GroupInformation()
               {
                   GroupName = grp.Key,
                   GroupCount = grp.Count()
               };

我得到以下SQL查询(取自SQL Profiler):

SELECT 
    1 AS [C1], 
    [GroupBy1].[K1] AS [Type], 
    [GroupBy1].[A1] AS [C2]
    FROM ( SELECT 
        [Extent1].[Type] AS [K1], 
        COUNT(1) AS [A1]
        FROM [dbo].[AccountingTransaction] AS [Extent1]
        GROUP BY [Extent1].[Type]
    )  AS [GroupBy1]

到现在为止还挺好.

如果我将我的LINQ查询更改为:

// Sample # 2
groupingInfo = context.AccountingTransaction.
                 GroupBy(a => a.Type).
                 Select(grp => new GroupInformation()
                               {
                                   GroupName = grp.Key,
                                   GroupCount = grp.Count()
                               });

它产生完全相同的SQL查询.我感觉合理.

这是有趣的部分...如果我将我的LINQ查询更改为:

// Sample # 3
IEnumerable<AccountingTransaction> accounts;
IEnumerable<IGrouping<object, AccountingTransaction>> groups;
IEnumerable<GroupInformation> groupingInfo;

accounts = context.AccountingTransaction;
groups = accounts.GroupBy(a => a.Type);
groupingInfo = groups.Select(grp => new GroupInformation()
                  {
                      GroupName = grp.Key,
                      GroupCount = grp.Count()
                  });

执行以下SQL(我从实际查询中删除了一些字段,但表中的所有字段(~15个字段)都包含在查询中,两次):

SELECT 
    [Project2].[C1] AS [C1], 
    [Project2].[Type] AS [Type], 
    [Project2].[C2] AS [C2], 
    [Project2].[Id] AS [Id], 
    [Project2].[TimeStamp] AS [TimeStamp], 
    -- <snip>
    FROM ( SELECT 
        [Distinct1].[Type] AS [Type], 
        1 AS [C1], 
        [Extent2].[Id] AS [Id], 
        [Extent2].[TimeStamp] AS [TimeStamp], 
        -- <snip>
        CASE WHEN ([Extent2].[Id] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C2]
        FROM   (SELECT DISTINCT 
            [Extent1].[Type] AS [Type]
            FROM [dbo].[AccountingTransaction] AS [Extent1] ) AS [Distinct1]
        LEFT OUTER JOIN [dbo].[AccountingTransaction] AS [Extent2] ON [Distinct1].[Type] = [Extent2].[Type]
    )  AS [Project2]
    ORDER BY [Project2].[Type] ASC, [Project2].[C2] ASC

为什么生成的SQL如此不同？毕竟,执行完全相同的代码,只是示例#3使用中间变量来完成相同的工作!

另外,如果我这样做:

Console.WriteLine(groupingInfo.ToString());

对于示例#1和示例#2,我得到了与SQL事件探查器捕获的完全相同的查询,但对于示例#3,我得到:

System.Linq.Enumerable+WhereSelectEnumerableIterator`2[System.Linq.IGrouping`2[System.Object,TestLinq.AccountingTransaction],TestLinq.GroupInformation]

有什么不同？如果我在多个指令中拆分LINQ查询,为什么我不能获得LINQ生成的SQL查询？

ulitmate的目标是能够在运行时向查询(Where,OrderBy等)添加运算符.

顺便说一下,我在EF 4.0和EF 6.0中看到过这种行为.

谢谢您的帮助.

Answer 1

究其原因是因为在你的第三次尝试你指的是accounts因为IEnumerable<AccountingTransaction>这将导致使用调用查询LINQ到对象(Enumerable.GroupBy和Enumerable.Select)

另一方面,在第一次和第二次尝试中,引用AccountingTransaction保留为,IQueryable<AccountingTransaction>并且将使用Linq-To-Entities执行查询,然后Linq-To-Entities将其转换为适当的SQL语句.