如何使用LINQ和EF6编写外部联接？

Question

我有三个表Exam，Test和UserTest。

CREATE TABLE [dbo].[Exam] (
    [ExamId]                      INT            IDENTITY (1, 1) NOT NULL,
    [SubjectId]                   INT            NOT NULL,
    [Name]                        NVARCHAR (50)  NOT NULL,
    [Description]                 NVARCHAR (MAX) NOT NULL,
    CONSTRAINT [PK_Exam] PRIMARY KEY CLUSTERED ([ExamId] ASC),
    CONSTRAINT [FK_ExamSubject] FOREIGN KEY ([SubjectId]) REFERENCES [dbo].[Subject] ([SubjectId]),
    CONSTRAINT [FK_Exam_ExamType] FOREIGN KEY ([ExamTypeId]) REFERENCES [dbo].[ExamType] ([ExamTypeId])
);

CREATE TABLE [dbo].[Test] (
    [TestId]      INT            IDENTITY (1, 1) NOT NULL,
    [ExamId]      INT            NOT NULL,
    [Title]       NVARCHAR (100) NULL,
    [Status]      INT            NOT NULL,
    [CreatedDate] DATETIME       NOT NULL,
    CONSTRAINT [PK_Test] PRIMARY KEY CLUSTERED ([TestId] ASC),
    CONSTRAINT [FK_TestExam] FOREIGN KEY ([ExamId]) REFERENCES [dbo].[Exam] ([ExamId])
);

CREATE TABLE [dbo].[UserTest] (
    [UserTestId]              INT            IDENTITY (1, 1) NOT NULL,
    [UserId]                  NVARCHAR (128) NOT NULL,
    [TestId]                  INT            NOT NULL,
    [Result]                  INT            NULL
    CONSTRAINT [PK_UserTest] PRIMARY KEY CLUSTERED ([UserTestId] ASC),
    CONSTRAINT [FK_UserTestTest] FOREIGN KEY ([TestId]) REFERENCES [dbo].[Test] ([TestId])
);

考试可以进行许多测试，而用户可以尝试多次。

如何使用扩展方法语法编写LINQ语句，使我能够看到UserId == 1的以下内容（我在Where子句中假定UserId == 1）：

Exam       Test      Title           UserTestID  UserId     Result
1          1         1a               1           1          20 
1          1         1a               2           1          30
1          1         1a               3           1          40         
1          2         1b               4           1          98 
1          3         1c               5           1          44
2          4         2a
2          5         2b               6           1          12

或者，如果UserId == 2：

Exam       Test      Title           UserTestID  UserId     Result
1          1         1a               7           2          27  
1          2         1b        
1          3         1c               8           2          45
2          4         2a
2          5         2b        

或者，如果UserId为null

Exam       Test      Title           UserTestID  UserId     Result
1          1         1a        
1          2         1b
1          3         1c  
2          4         2a
2          5         2b   

请注意，由于我收到的建议，此问题已作了一些更改。现在有悬赏，我希望我可以接受一个快速的答案。

Answer 1

如果您的Test实体有一个UserTests集合，则可以使用以下查询：

string userId = "1";
var result = context.Tests
    .SelectMany(t => t.UserTests
        .Where(ut => ut.UserId == userId)
        .DefaultIfEmpty()
        .Select(ut => new
        {
            ExamId = t.ExamId,
            TestId = t.TestId,
            Title = t.Title,
            UserTestId = (int?)ut.UserTestId,
            UserId = ut.UserId,
            Result = ut.Result
        }))
    .OrderBy(x => x.ExamId)
    .ThenBy(x => x.TestId)
    .ThenBy(x => x.UserTestId)
    .ToList();

使用DefaultIfEmpty()此处可以确保给定LEFT OUTER JOIN始终拥有至少一个UserTest实体（可能是null）Test。UserTest例如，将-之类的不可为null的属性UserTestId强制转换为可int?为null的类型非常重要，否则，您可能会遇到一个例外，即NULL从数据库返回的值不能存储为不可为null的.NET类型。

如果您的实体中没有UserTests集合并且不想要集合，则Test可以使用a GroupJoin作为替代，基本上可以通过以下方式使外部连接两个表TestId：

string userId = "1";
var result = context.Tests
    .GroupJoin(context.UserTests.Where(ut => ut.UserId == userId),
        t => t.TestId,
        ut => ut.TestId,
        (t, utCollection) => new
        {
            Test = t,
            UserTests = utCollection
        })
    .SelectMany(x => x.UserTests
        .DefaultIfEmpty()
        .Select(ut => new
        {
            ExamId = x.Test.ExamId,
            TestId = x.Test.TestId,
            Title = x.Test.Title,
            UserTestId = (int?)ut.UserTestId,
            UserId = ut.UserId,
            Result = ut.Result
        }))
    .OrderBy(x => x.ExamId)
    .ThenBy(x => x.TestId)
    .ThenBy(x => x.UserTestId)
    .ToList();