[英]NULL handling in dbcontext and objectcontext
我有这个简单的LINQ查询
from e in Employees
where e.DesignationID !=558
select e
这里的DesignationID
是一个可以为空的字段:
在objectcontext
,查询被转换为:
SELECT
[Extent1].[EmployeeID] AS [EmployeeID],
[Extent1].[EmployeeCode] AS [EmployeeCode],
[Extent1].[EmployeeName] AS [EmployeeName],
[Extent1].[DesignationID] AS [DesignationID]
FROM [dbo].[setupEmployees] AS [Extent1]
WHERE 558 <> [Extent1].[DesignationID]
虽然dbcontext
的相同查询被翻译为:
SELECT
[Extent1].[EmployeeID] AS [EmployeeID],
[Extent1].[EmployeeCode] AS [EmployeeCode],
[Extent1].[EmployeeName] AS [EmployeeName],
[Extent1].[DesignationID] AS [DesignationID]
FROM [dbo].[setupEmployees] AS [Extent1]
WHERE NOT ((558 = [Extent1].[DesignationID]) AND ([Extent1].[DesignationID] IS NOT NULL))
为什么objectcontext
不同于dbcontext
处理NULL?
这种行为是可配置的,所以很可能是一个不同的默认值(我不知道为什么默认值不同)。
该控件由DbContextConfiguration.UseDatabaseNullSemantics属性提供:
获取或设置一个值,该值指示在比较两个操作数时是否显示数据库空语义,这两个操作数都可能为空。 默认值为false 。 例如(operand1 == operand2)将被翻译为:(operand1 = operand2)如果UseDatabaseNullSemantics分别为true(((operand1 = operand2)AND(NOT(operand1 IS NULL或operand2 IS NULL)))OR((operand1 IS) NULL)AND(operand2 IS NULL)))如果UseDatabaseNullSemantics为false。
要获得与第一个示例中相同的转换,应将其设置为true
(例如,在db上下文构造函数中):
public YourDbContext()
{
// ...
this.Configuration.UseDatabaseNullSemantics = true;
}
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.