LINQ左连接只有具有最大值的行

Question

我有一个表 ProjectInformation 作为（左表）

ProjectID int
{fields}

另一个表，ProjectUpdates（右表）。此表有多个记录，每月添加 1 个 ProjectID。

ProjectID int
CreateDate date
{other fields that have records}

它们的关系是 1-M。 样本数据

ProjectInformation
{ ProjectID = 1, DataA = "ABC"}
{ ProjectID = 2, DataA = "DEF"}
{ ProjectID = 3, DataA = "GHI"}

ProjectUpdates
{ProjectID = 1, CreateDate = "24/2/2014", DataB = "JKL"}
{ProjectID = 1, CreateDate = "25/1/2014", DataB = "MNL"}
{ProjectID = 1, CreateDate = "23/12/2014", DataB = "PQR"}
{ProjectID = 1, CreateDate = "23/11/2014", DataB = "STU"}
{ProjectID = 2, CreateDate = "24/2/2014", DataB = "VWX"}
{ProjectID = 2, CreateDate = "24/1/2014", DataB = "YZA"}
{ProjectID = 3, CreateDate = "21/12/2014", DataB = "BCD"}
{ProjectID = 3, CreateDate = "24/11/2014", DataB = "EFG"}
{ProjectID = 3, CreateDate = "24/10/2014", DataB = "HIJ"}
{ProjectID = 3, CreateDate = "24/8/2014", DataB = "KLM"}
{ProjectID = 3, CreateDate = "24/6/2014", DataB = "NOP"}

我希望我的 LINQ 查询返回以下数据（每个项目 1 行）

条件：左表中的行将与项目中 CreateDate 值最大的右表连接。

{ProjectID = 1, CreateDate = "24/2/2014", DataA ="ABC", DataB = "JKL"}
{ProjectID = 2, CreateDate = "24/2/2014", DataA ="DEF", DataB = "VWX"}
{ProjectID = 3, CreateDate = "21/12/2014", DataA ="GHI", DataB = "BCD"}

Answer 1

试试这个：-

var result = (from pi in projectInfo
              join pu in projectupdates
              on pi.ProjectID equals pu.ProjectID into g
              let data = g.OrderByDescending(x => DateTime.ParseExact(x.CreateDate, 
                           "dd/M/yyyy", CultureInfo.InvariantCulture)).FirstOrDefault()
              select new
              {
                  ProjectID = data.ProjectID,
                  CreatedDate = data.CreateDate,
                  DataA = pi.DataA,
                  DataB = data.DataB
               }).ToList();

Working Fiddle.

请注意，由于我使用的是FirstOrDefault，因此您需要在访问其属性之前检查数据是否不为空。

Answer 2

您可以通过多种方式做到这一点。其中一种方法是：

var result = (from pi in projectInformations
              join pu in projectUpdates on pi.ProjectID equals pu.ProjectID into tpu
              from t in tpu.OrderByDescending(c => c.CreateDate).Take(1)
              select new { pi.ProjectID, pi.DataA, t.CreateDate, t.DataB }).ToList();

Answer 3

您可以使用从 ProjectInformation 到 ProjectUpdate 和 OrderByDescending/FirstOrDefault 的 one to many 关联。

public class ProjectInformation {
  public int ProjectID { get; set; }
  public virtual ICollection<ProjectUpdate> ProjectUpdates { get; set; }
}

db.Project
  .Select(p => new {
    ProjectID = p.ProjectID,
    MostRecentProjectUpdate = p.ProjectUpdates.OrderByDescending(u => u.CreateDate).FirstOrDefault()
});

注意：如果 CreateDate 是字符串而不是数据库中的日期，则使用 OrderByDescending 可能会很棘手。