[英]What is the difference between these LINQ queries
我一直在使用一些LINQ over Entities,我得到了奇怪的結果,我想得到解釋......
給出以下LINQ查詢,
// 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查詢(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]
到現在為止還挺好。
如果我將我的LINQ查詢更改為:
// Sample # 2
groupingInfo = context.AccountingTransaction.
GroupBy(a => a.Type).
Select(grp => new GroupInformation()
{
GroupName = grp.Key,
GroupCount = grp.Count()
});
它產生完全相同的SQL查詢。 我感覺合理。
這是有趣的部分...如果我將我的LINQ查詢更改為:
// 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()
});
執行以下SQL(我從實際查詢中刪除了一些字段,但表中的所有字段(~15個字段)都包含在查詢中,兩次):
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
為什么生成的SQL如此不同? 畢竟,執行完全相同的代碼,只是示例#3使用中間變量來完成相同的工作!
另外,如果我這樣做:
Console.WriteLine(groupingInfo.ToString());
對於示例#1和示例#2,我得到了與SQL事件探查器捕獲的完全相同的查詢,但對於示例#3,我得到:
System.Linq.Enumerable+WhereSelectEnumerableIterator`2[System.Linq.IGrouping`2[System.Object,TestLinq.AccountingTransaction],TestLinq.GroupInformation]
有什么不同? 如果我在多個指令中拆分LINQ查詢,為什么我不能獲得LINQ生成的SQL查詢?
ulitmate的目標是能夠在運行時向查詢(Where,OrderBy等)添加運算符。
順便說一下,我在EF 4.0和EF 6.0中看到過這種行為。
謝謝您的幫助。
原因是因為在您的第三次嘗試中,您將accounts
稱為IEnumerable<AccountingTransaction>
,這將導致使用Linq-To-Objects ( Enumerable.GroupBy
和Enumerable.Select
)調用查詢
另一方面,在第一次和第二次嘗試中,對AccountingTransaction
的引用保留為IQueryable<AccountingTransaction>
並且將使用Linq-To-Entities執行查詢,然后Linq-To-Entities將其轉換為適當的SQL語句。
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