----------- [知识分享] LINQ TO SQL Null 查询 ---------------

在论坛里不止一次看到有网友提问关于LINQ NULL查询的问题了，现以微软NorthWind 数据库为例总结一下：
如查询这样一句SQL ，用LINQ如何实现？

SELECT *

FROM [Orders] AS [t0]

WHERE ([t0].[ShippedDate]) IS NULL

方法一:

from o in Orders

where o.ShippedDate==null

select o

对应的Lamda表达式为:

Orders.Where (o => (o.ShippedDate == (DateTime?)null))

对应的SQL语句为:

SELECT [t0].[OrderID], [t0].[CustomerID], [t0].[EmployeeID], [t0].[OrderDate], [t0].[RequiredDate], [t0].[ShippedDate], [t0].[ShipVia], [t0].[Freight], [t0].[ShipName], [t0].[ShipAddress], [t0].[ShipCity], [t0].[ShipRegion], [t0].[ShipPostalCode], [t0].[ShipCountry]

FROM [Orders] AS [t0]

WHERE [t0].[ShippedDate] IS NULL

方法二:

from o in Orders

where Nullable<DateTime>.Equals(o.ShippedDate,null)

select o

对应的Lamda表达式为:

Orders.Where (o => Object.Equals (o.ShippedDate, null))

对应的SQL语句为:

SELECT [t0].[OrderID], [t0].[CustomerID], [t0].[EmployeeID], [t0].[OrderDate], [t0].[RequiredDate], [t0].[ShippedDate], [t0].[ShipVia], [t0].[Freight], [t0].[ShipName], [t0].[ShipAddress], [t0].[ShipCity], [t0].[ShipRegion], [t0].[ShipPostalCode], [t0].[ShipCountry]

FROM [Orders] AS [t0]

WHERE [t0].[ShippedDate] IS NULL

方法三:

from o in Orders

where !o.ShippedDate.HasValue

select o

对应的Lamda表达式为:

Orders.Where (o => !(o.ShippedDate.HasValue))

对应的SQL语句为:

SELECT [t0].[OrderID], [t0].[CustomerID], [t0].[EmployeeID], [t0].[OrderDate], [t0].[RequiredDate], [t0].[ShippedDate], [t0].[ShipVia], [t0].[Freight], [t0].[ShipName], [t0].[ShipAddress], [t0].[ShipCity], [t0].[ShipRegion], [t0].[ShipPostalCode], [t0].[ShipCountry]

FROM [Orders] AS [t0]

WHERE NOT ([t0].[ShippedDate] IS NOT NULL)

方法四:

from o in Orders

where o.ShippedDate.Value==(DateTime?)null

select o

对应的Lamda表达式为:

Orders.Where (o => ((DateTime?)(o.ShippedDate.Value) == (DateTime?)null))

对应的SQL语句为:

SELECT [t0].[OrderID], [t0].[CustomerID], [t0].[EmployeeID], [t0].[OrderDate], [t0].[RequiredDate], [t0].[ShippedDate], [t0].[ShipVia], [t0].[Freight], [t0].[ShipName], [t0].[ShipAddress], [t0].[ShipCity], [t0].[ShipRegion], [t0].[ShipPostalCode], [t0].[ShipCountry]

FROM [Orders] AS [t0]

WHERE ([t0].[ShippedDate]) IS NULL

方法五:

from o in Orders

where  System.Data.Linq.SqlClient.SqlMethods.Equals(o.ShippedDate.Value,null)

select o

对应的Lamda表达式为:

Orders.Where (o => Object.Equals (o.ShippedDate.Value, null))

对应的SQL语句为:

SELECT [t0].[OrderID], [t0].[CustomerID], [t0].[EmployeeID], [t0].[OrderDate], [t0].[RequiredDate], [t0].[ShippedDate], [t0].[ShipVia], [t0].[Freight], [t0].[ShipName], [t0].[ShipAddress], [t0].[ShipCity], [t0].[ShipRegion], [t0].[ShipPostalCode], [t0].[ShipCountry]

FROM [Orders] AS [t0]

WHERE ([t0].[ShippedDate]) IS NULL

以上方法均只在LINQ TO SQL内验证实现，LINQ TO EF未验证。

具体参见myblog: http://blog.csdn.net/q107770540/article/details/7348384