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今天拿Entity Framework改写早期的一个项目，涉及到对NULL值处理的时候，遇到了点问题，就是如何查询数据库中某字段为NULL的记录，写了几个测试用的例子，然后用SQL Server Profiler抓SQL。

测试1：直接使用null

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

对应的SQL是：

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

测试2：使用等于null的变量：

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

对应的SQL是：

-- 使用了值为NULL的变量，但是用了等号，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 

测试3：加判断

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

  对应的SQL是：

-- 无语了，这也整得太复杂了吧，显然得不到正确结果 
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 

测试4：另一种方式加判断

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

对应的SQL是：

--继续无语，比刚才的还复杂，同样得不到正确结果 
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 

晕倒，看来把Linq的思想直接往EF上套是行不通的，某些情况下差异还是挺大的。