c# - SQL 查询 CAST 到 int + 1 到 LINQ

Question

我正在为以下 linq to sql 查询绞尽脑汁。想法是获得与所有 REG_CODE 项目相比的下一个最小整数。该字段 (REG_CODE) 是一个 varchar(10) 字段。我正在尝试将以下 tsql 转换为 linq-to-entities (EF 6.0):

SELECT TOP 1 CAST( [Extent1].[REG_CODE] AS int) + 1 
    FROM [dbo].[Notifications] [Extent1]
WHERE NOT ([Extent1].[REG_CODE] LIKE N'%~[a-z]%' ESCAPE N'~') AND (1 = (ISNUMERIC([Extent1].[REG_CODE]))) AND
      NOT EXISTS(SELECT * FROM [dbo].[Notifications] t2 WHERE CAST( t2.REG_CODE AS int) = CAST( [Extent1].[REG_CODE] AS int) + 1 )
ORDER BY [Extent1].[REG_CODE]

(注意 +1 的，我要下一部分)设计并不是那么棒。字段 [REG_CODE] 应该是一个整数字段，但现在不是，也不会很快。

这个:

float notificationMaxRegCodeNumeric =
    db.Notifications.Where(not => not.Reg_Code != null && !not.Reg_Code.Contains("[a-z]") && SqlFunctions.IsNumeric(not.Reg_Code) == 1)
        .OrderByDescending(not => not.Reg_Code)
        .Select(not => not.Reg_Code)
        .Cast<int>()
        .Max();

成功转换为:

SELECT MAX(CAST( [Extent1].[REG_CODE] AS int)) AS[A1]
FROM[dbo].[Notifications] AS[Extent1]
WHERE ([Extent1].[REG_CODE] IS NOT NULL) AND(NOT([Extent1].[REG_CODE] LIKE N'%~[a-z]%' ESCAPE N'~')) AND(1 = (ISNUMERIC([Extent1].[REG_CODE])))

到目前为止我有:

int nextNotificationMaxRegCodeNumericInt = db.Notifications.Where(not =>
    not.Reg_Code != null && !not.Reg_Code.Contains("[a-z]") &&
    SqlFunctions.IsNumeric(not.Reg_Code) == 1 &&
    db.Notifications.Any(klainternal => not.Reg_Code.Cast<int>() == klainternal.Reg_Code.Cast<int>())
    )
.OrderByDescending(not => not.Reg_Code)
.Select(not => not.Reg_Code)
.Cast<int>();

但它抛出:

DbExpressionBinding requires an input expression with a collection ResultType`.

还有 Convert.ToInt32() 正在抛出:

linq to entities does not recognize the method 'int32 toint32(system.string)' method`

(.Max() 与我正在寻找的内容并不相关，但它在查询的工作部分)

有什么建议吗？

Answer 1

首先，恭喜您找到Cast<T>()诡计!这似乎是唯一开箱即用的 EF6 类型转换方式 string到别的东西 - 所有其他尝试，如 (int)(object)stringValue或 Convert.ToInt32(stringValue)只是因为不受支持而被阻止。

但请注意 Cast<T>()为 IEnumerable 定义了方法和 IQueryable结果分别是IEnumerable<T>和 IQueryable<T> ，即处理序列并产生序列。它出现在string因为string是IEnumerable<char> ，因此 IEnumerable ，但这不是我们需要的。

所以诀窍是始终使用投影 ( Select ) + Cast做转换。将它应用于您的查询会导致如下结果:

int nextNotificationMaxRegCodeNumericInt = db.Notifications
    .Where(n => n.Reg_Code != null &&
        !n.Reg_Code.Contains("[a-z]") &&
        SqlFunctions.IsNumeric(n.Reg_Code) == 1)
    .Select(n => n.Reg_Code).Cast<int>() // <--
    .Select(reg_Code => reg_Code + 1)
    .Where(reg_Code => !db.Notifications.Select(n => n.Reg_Code).Cast<int>() // <--
        .Contains(reg_Code))
    .OrderByDescending(reg_Code => reg_Code)
    .FirstOrDefault();

转换为

SELECT TOP (1)
    [Project1].[C1] AS [C1]
    FROM ( SELECT
         CAST( [Extent1].[Reg_Code] AS int) + 1 AS [C1]
        FROM [dbo].[Notifications] AS [Extent1]
        WHERE ([Extent1].[Reg_Code] IS NOT NULL) AND ( NOT ([Extent1].[Reg_Code] LIKE N'%~[a-z]%' ESCAPE N'~')) AND (1 = (ISNUMERIC([Extent1].[Reg_Code])))
    )  AS [Project1]
    WHERE  NOT EXISTS (SELECT
        1 AS [C1]
        FROM [dbo].[Notifications] AS [Extent2]
        WHERE  CAST( [Extent2].[Reg_Code] AS int) = [Project1].[C1]
    )
    ORDER BY [Project1].[C1] DESC