Question

我有一个像这样看起来（简化）的数据库：

var reports = new[] {
    new { ReportId = 1, Title = "Report A" },
    new { ReportId = 2, Title = "Report B" },
};

var testCases = new[] {
    new { TestId = 1, Title = "Test A" },
    new { TestId = 2, Title = "Test B" },
    new { TestId = 3, Title = "Test C" },
    new { TestId = 4, Title = "Test D" },
    new { TestId = 5, Title = "Test E" },
};

var testRuns = new[] {
    new { TestId = 1, ReportId = 1 },
    new { TestId = 2, ReportId = 1 },
    new { TestId = 1, ReportId = 2 },
    new { TestId = 2, ReportId = 2 },
    new { TestId = 3, ReportId = 2 },
    new { TestId = 4, ReportId = 2 },
};

因此，我希望获得一个testCases列表，其中包含相应的reports，即：

Test A => [Report A, Report B]
Test B => [Report A, Report B]
Test C => [Report B]
Test D => [Report B]
Test E => []

我不确定如何在SQL或LINQ中表达它。我想我需要一个组连接或左外连接，或者它被调用，但我还无法找出正确的语法。我尝试过类似的东西，但仍然错过了一部分：

var result = tests.GroupJoin(reports, t => t.TestId, ???, (t, rs) => new { Test = t, Reports = rs });

或者可能有一种完全不同的方式来制定该查询。

编辑：并非testCases中引用testRuns中的每个条目。

Answer 1

如果你可以在没有GroupJoin的情况下生活，你可以使用2个简单连接和GroupBy这样做：

var temp = from c in testCases
           join ru in testRuns
           on c.TestId equals ru.TestId into left
           from l in left
           join re in reports 
           on l.ReportId equals re.ReportId into foo
           from f in foo
           select new {
                Test = c.Title,
                Report = f
           };
//Dump only in LinqPad!            
temp.GroupBy(x => x.Test).Dump();

/编辑：如果您还想要空结果，则需要使用DefaultIfEmpty()：

public class Report
{
    public int ReportId { get; set; }
    public string Title { get; set; }
}

public class TestCase
{
    public int TestId { get; set; }
    public string Title { get; set; }
}

public class TestRun
{
    public int TestId { get; set; }
    public int ReportId { get; set; }
}
var temp = from c in testCases
           join ru in testRuns
           on c.TestId equals ru.TestId into left
           from l in left.DefaultIfEmpty(new TestRun())
           join re in reports 
           on l.ReportId equals re.ReportId into foo
           from f in foo.DefaultIfEmpty()
           select new {
                Test = c.Title,
                Report = f 
           };

temp.Dump();

演示代码：http://share.linqpad.net/t3osbj.linq
使用DefaultIfempty进行强类型演示：http://share.linqpad.net/fsov4p.linq
使用DefaultIfEmpty：http://share.linqpad.net/7k2lq4.linq

Answer 2

我认为你不需要小组加入。这将为您提供所需。结果将包含测试ID以及与之关联的报告。

var result = testRuns.GroupBy(tr => tr.TestID)
                     .Select(t => new { 
                                       TestID = t.Key,
                                       Reports = t.Select(r => r.ReportID).ToList()
                                      });

如何制定GroupJoin（）语句

2 个答案: