Question

我想知道为什么生成的SQL检查不可为空的列上的空值（列Value是非空浮点数），

var query = _context.Events
  .Select(e => new
  {
    eventId = e.EventId,
    data = e.Data.Take(10).Select(x => new
    {
      name = x.Name,
      value = Math.Round(x.Value,1),
    })
  });

生成以下SQL代码：

SELECT 
    [Project2].[EventId] AS [EventId], 
    [Project2].[DeviceId] AS [DeviceId], 
    [Project2].[TimeEnd] AS [TimeEnd], 
    [Project2].[C2] AS [C1], 
    [Project2].[EventId1] AS [EventId1], 
    [Project2].[Name] AS [Name], 
    [Project2].[C1] AS [C2]
    FROM ( SELECT 
        [Limit1].[EventId] AS [EventId], 
        [Limit1].[TimeEnd] AS [TimeEnd], 
        [Limit1].[DeviceId] AS [DeviceId], 
        [Extent2].[Name] AS [Name], 
        [Extent2].[EventId] AS [EventId1], 
        CASE WHEN ([Extent2].[Value] IS NULL) THEN CAST(NULL AS float) ELSE ROUND([Extent2].[Value], 1) END AS [C1], 
        CASE WHEN ([Extent2].[Value] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C2]
        FROM   (SELECT [Project1].[EventId] AS [EventId], [Project1].[TimeEnd] AS [TimeEnd], [Project1].[DeviceId] AS [DeviceId]
            FROM ( SELECT 
                [Extent1].[EventId] AS [EventId], 
                [Extent1].[TimeEnd] AS [TimeEnd], 
                [Extent1].[DeviceId] AS [DeviceId]
                FROM [dbo].[Events] AS [Extent1]
                WHERE ([Extent1].[DeviceId] = 1)
            )  AS [Project1]
            ORDER BY [Project1].[TimeEnd] DESC ) AS [Limit1]
        LEFT OUTER JOIN [dbo].[Octaves] AS [Extent2] ON [Limit1].[EventId] = [Extent2].[EventId]
    )  AS [Project2]
    ORDER BY [Project2].[TimeEnd] DESC, [Project2].[EventId] ASC, [Project2].[C2] ASC

如果我删除了两个CASE WHEN ([Extent2].[Value] IS NULL)并离开ROUND([Limit2].[Value], 1) AS [C1]并删除了SQL Server Management中的ORDER BY Project2].[C2] ASC，则查询速度会加快。

Answer 1

有一个LEFT OUTER JOIN，因此值（在查询中）绝对可以为NULL（即使基础表定义不可为空）。

请参阅http://docs.oracle.com/javadb/10.8.3.0/ref/rrefsqlj18922.html（对于Oracle - 但相同的基本模式适用于所有主要SQL供应商）。

在这种情况下，它有点无意义，因为ROUND（NULL，1）无论如何都会返回NULL（好吧，它会在SQL Server上）。但是，由于无法控制它生成的SQL，因此您无法做很多事情。

EF6生成不需要的SQL查询

1 个答案: