LINQ中的多个左连接
本文关键字:连接 LINQ | 更新日期: 2023-09-27 18:18:15
我正试图将SQL查询转换为LINQ,并且我在获得语法正确方面遇到麻烦。我的原始(工作)SQL查询是:
SELECT a.PersonnelNumber,
a.LastName,
a.FirstName,
a.MiddleInitial,
b.Title,
b.Division,
b.Section,
b.Unit,
d.PersonnelNumber AS SupervisorPersonnelNumber
FROM Person a
JOIN Position b ON a.PositionID = b.PositionID
LEFT JOIN Position c ON b.SupervisorPositionID = c.PositionID
LEFT JOIN Person d ON c.PositionID = d.PositionID
我把它转换成下面的LINQ:
var query = from a in ctx.People
from b in ctx.Positions.Where(b => a.PositionID == b.PositionID)
from c in ctx.Positions.Where(c => b.SupervisorPositionID == c.PositionID).DefaultIfEmpty()
from d in ctx.People.Where(d => c.PositionID == d.PositionID).DefaultIfEmpty()
select new {
a.PersonnelNumber,
a.LastName,
a.FirstName,
a.MiddleInitial,
b.Title,
b.Division,
b.Section,
b.Unit,
SupervisorPersonnelNumber = d.PersonnelNumber
};
返回的结果比我预期的要多(20000+而不是~1100),所以我查看生成的SQL:
SELECT
[Extent2].[PositionID] AS [PositionID],
[Extent1].[PersonnelNumber] AS [PersonnelNumber],
[Extent1].[LastName] AS [LastName],
[Extent1].[FirstName] AS [FirstName],
[Extent1].[MiddleInitial] AS [MiddleInitial],
[Extent2].[Title] AS [Title],
[Extent2].[Division] AS [Division],
[Extent2].[Section] AS [Section],
[Extent2].[Unit] AS [Unit],
[Extent4].[PersonnelNumber] AS [PersonnelNumber1]
FROM [dbo].[Person] AS [Extent1]
INNER JOIN [dbo].[Position] AS [Extent2] ON [Extent1].[PositionID] = [Extent2].[PositionID]
LEFT OUTER JOIN [dbo].[Position] AS [Extent3] ON [Extent2].[SupervisorPositionID] = [Extent3].[PositionID]
LEFT OUTER JOIN [dbo].[Person] AS [Extent4] ON ([Extent3].[PositionID] = [Extent4].[PositionID]) OR (([Extent3].[PositionID] IS NULL) AND ([Extent4].[PositionID] IS NULL))
这是什么导致我的问题的最后一行:
LEFT OUTER JOIN [dbo].[Person] AS [Extent4] ON ([Extent3].[PositionID] = [Extent4].[PositionID]) OR (([Extent3].[PositionID] IS NULL) AND ([Extent4].[PositionID] IS NULL))
我不确定为什么要添加额外的OR
子句,并删除它返回所需的结果。
如果有帮助,Position
表与Person
有(有效的,尽管不是强制的)1:1的关系,Position
与自身有关系:PositionID
是FK到SupervisorPositionID
CREATE TABLE [dbo].[Position](
[PositionID] [int] IDENTITY(1,1) NOT NULL,
[PositionNumber] [varchar](8) NULL,
[Title] [varchar](40) NULL,
[Division] [varchar](40) NULL,
[Section] [varchar](40) NULL,
[Unit] [varchar](40) NULL,
[SupervisorPositionID] [int] NULL,
)
CREATE TABLE [dbo].[Person](
[PersonID] [int] IDENTITY(1,1) NOT NULL,
[PersonnelNumber] [varchar](8) NOT NULL,
[LastName] [varchar](40) NULL,
[FirstName] [varchar](40) NULL,
[MiddleInitial] [char](1) NULL,
[PositionID] [int] NULL,
)
为什么OR (([Extent3].[PositionID] IS NULL) AND ([Extent4].[PositionID] IS NULL))
被附加到这一行的末尾,我能做些什么来修复它?
您需要更改DbContext上的设置。属性是UseDatabaseNullSemantics,您需要将其设置为false
context.Configuration.UseDatabaseNullSemantics = false;
我目前没有任何工具来检查这是否会产生所需的输出,但我认为它应该足够接近:
from a in Persons
join b in Positions on a.PositionID equals b.PositionID
join c in Positions on b.SupervisorPositionID equals c.PositionID into SupervisorsPositions
from c in SupervisorsPositions.DefaultIfEmpty()
join d in Persons on c.PositionID equals d.PositionID into PersonalNumbers
from d in PersonalNumbers.DefaultIfEmpty()
select new {
a.PersonnelNumber,
a.LastName,
a.FirstName,
a.MiddleInitial,
b.Title,
b.Division,
b.Section,
b.Unit,
SupervisorPersonnelNumber = d.PersonnelNumber
}
这个查询将产生:
SELECT [t0].[PersonnelNumber], [t0].[LastName], [t0].[FirstName], [t0].[MiddleInitial], [t1].[Title], [t1].[Division], [t1].[Section], [t1].[Unit], [t3].[PersonnelNumber] AS [SupervisorPersonnelNumber]
FROM [Person] AS [t0]
INNER JOIN [Position] AS [t1] ON [t0].[PositionID] = ([t1].[PositionID])
LEFT OUTER JOIN [Position] AS [t2] ON [t1].[SupervisorPositionID] = ([t2].[PositionID])
LEFT OUTER JOIN [Person] AS [t3] ON ([t2].[PositionID]) = [t3].[PositionID]
由T-SQL语言规范LEFT OUTER JOIN等于LEFT JOIN。INNER JOIN = JOIN
以便查询产生所需的结果。有关T-SQL中Join类型的更多信息,请参阅答案。