Entity Framework對NULL值的處理

今天拿Entity Framework改寫早期的一個項目，涉及到對NULL值處理的時候，遇到了點問題，就是如何查詢數據庫中某字段爲NULL的記錄，寫了幾個測試用的例子，然後用SQL Server Profiler抓SQL。

測試1：直接使用null

var clients = ctx.Clients.Where(c => c.ParentGUID == null); 

對應的SQL是：

-- 執行正常 
SELECT  
[Extent1].[GUID] AS [GUID],  
[Extent1].[Name] AS [Name],  
[Extent1].[ParentGUID] AS [ParentGUID] 
FROM [dbo].[Clients] AS [Extent1] 
WHERE [Extent1].[ParentGUID] IS NULL 

測試2：使用等於null的變量：

string parentGuid = null; 
var clients = ctx.Clients.Where(c => c.ParentGUID == parentGuid); 

對應的SQL是：

-- 使用了值爲NULL的變量，但是用了等號，所以取不到值了 
exec sp_executesql N'SELECT  
[Extent1].[GUID] AS [GUID],  
[Extent1].[Name] AS [Name],  
[Extent1].[ParentGUID] AS [ParentGUID] 
FROM [dbo].[Clients] AS [Extent1] 
WHERE [Extent1].[ParentGUID] = @p__linq__0',N'@p__linq__0 varchar(8000)',@p__linq__0=NULL 

測試3：加判斷

string parentGuid = null; 
var clients = ctx.Clients.Where(c => c.ParentGUID == (string.IsNullOrEmpty(parentGuid) ? null : parentGuid)); 

  對應的SQL是：

-- 無語了，這也整得太複雜了吧，顯然得不到正確結果 
exec sp_executesql N'SELECT  
[Extent1].[GUID] AS [GUID],  
[Extent1].[Name] AS [Name],  
[Extent1].[ParentGUID] AS [ParentGUID] 
FROM [dbo].[Clients] AS [Extent1] 
WHERE [Extent1].[ParentGUID] = ( 
    CASE  
        WHEN ((@p__linq__0 IS NULL) OR (( CAST(LEN(@p__linq__0) AS int)) = 0))  
        THEN CAST(NULL AS varchar(1))  
        ELSE @p__linq__1  
    END 
)',N'@p__linq__0 nvarchar(4000),@p__linq__1 nvarchar(4000)',@p__linq__0=NULL,@p__linq__1=NULL 

測試4：另一種方式加判斷

string parentGuid = null; 
var clients = ctx.Clients.Where(c => string.IsNullOrEmpty(parentGuid) ? c.ParentGUID == null : c.ParentGUID == parentGuid); 

對應的SQL是：

--繼續無語，比剛纔的還複雜，同樣得不到正確結果 
exec sp_executesql N'SELECT  
[Extent1].[GUID] AS [GUID],  
[Extent1].[Name] AS [Name],  
[Extent1].[ParentGUID] AS [ParentGUID] 
FROM [dbo].[Clients] AS [Extent1] 
WHERE ( 
    CASE  
        WHEN ((@p__linq__0 IS NULL) OR (( CAST(LEN(@p__linq__0) AS int)) = 0))  
        THEN cast(0 as bit)  
         
        WHEN ([Extent1].[ParentGUID] = @p__linq__1)  
        THEN cast(1 as bit)  
         
        WHEN ([Extent1].[ParentGUID] <> @p__linq__1)  
        THEN cast(0 as bit)  
    END 
) = 1',N'@p__linq__0 nvarchar(4000),@p__linq__1 varchar(8000)',@p__linq__0=NULL,@p__linq__1=NULL 

暈倒，看來把Linq的思想直接往EF上套是行不通的，某些情況下差異還是挺大的。