LINQ 查询中的分组依据

Question

我有一个 DataTable，我想在其中执行 GroupBy 查询。

year    url      type     id
=============================
        someurl  image    0
2003             date     0
        someurl  image    1
2009             date     1

我已成功按 ID 进行分组，但无法选择“年份”和“网址”列。

这是我的查询：

var query = from row in table.AsEnumerable()
            let uri = row["uri"]
            group row by row.Field<int>("id") into grp
            orderby grp.Key
            select new
            {
                ID = grp.Key,
            };

如何选择年份和网址列？

提前致谢。

Answer 1

当您在 LINQ 中进行分组时，您的案例中的组 (grp) 是一个包含组中所有条目（案例中的数据行）的枚举。然后，您可以 foreach 条目或使用 Max、First 等聚合函数从可枚举中提取单个值

Answer 2

您可以按多个值进行分组。在您的案例中，按 id、年份和 url 分组
检查此处

    List<WebResource> webResources = new List<WebResource>();
    webResources.Add(new WebResource() { Id = 1, Type = "Image", Url = "url1", Year = 2001 });
    webResources.Add(new WebResource() { Id = 1, Type = "Text", Url = "url1", Year = 2001 });
    webResources.Add(new WebResource() { Id = 3, Type = "Image", Url = "url1", Year = 2001 });
    webResources.Add(new WebResource() { Id = 1, Type = "Text", Url = "url1", Year = 2002 });
    webResources.Add(new WebResource() { Id = 2, Type = "Image", Url = "url1", Year = 2002 });
    webResources.Add(new WebResource() { Id = 3, Type = "Text", Url = "url2", Year = 2002 });
    webResources.Add(new WebResource() { Id = 1, Type = "Image", Url = "url1", Year = 2002 });
    webResources.Add(new WebResource() { Id = 2, Type = "Text", Url = "url2", Year = 2002 });

    var output = webResources.GroupBy(webResource => new {webResource.Id, webResource.Year, webResource.Url })
        .Select(group => new {Key = group.Key, Count = group.Count()});

    foreach (var webResource in output)
    {
        Console.WriteLine("Id: " + webResource.Key.Id +" Url:" + webResource.Key.Url + " Year:" + webResource.Key.Year + "  Count:" + webResource.Count);
    }