高效的数据表分组依据

本文关键字：数据表 高效 | 更新日期: 2023-09-27 18:17:37

我想对一个数据表执行聚合查询以创建另一个数据表。我无法更改用于创建初始数据表的 SQL。

原始数据表：(一切都是整数(

TeamID | MemberID
-------|-----------
1      | 1
1      | 2
1      | 3
2      | 4
2      | 5

期望的结果：

TeamID | MemberIDCount
-------|--------------
1      | 3
2      | 2

如果是SQL，我可以做 

Select TeamID, Count(*) From Table Group By TeamID

但是在我的应用程序中，我知道如何处理这个问题的唯一方法是这样的：

Dictionary<int,int> d = new Dictionary<int,int>();
foreach (DataRow dr in dt.Rows)
{
    if (d.ContainsKey(dr.ID))
    {
        d[dr.ID] = d[dr.ID] + 1;
    }
    else
    {
        d.Add(dr.ID, 1);
    }
}

有没有更好的方法？

高效的数据表分组依据

你可以使用 Linq。

var result = from row in dt.AsEnumerable()
              group row by row.Field<int>("TeamID") into grp
               select new
                 {
                 TeamID = grp.Key,
                  MemberCount = grp.Count()
                  };
 foreach (var t in result)
     Console.WriteLine(t.TeamID + " " + t.MemberCount);

public DataTable GroupBy(string i_sGroupByColumn, string i_sAggregateColumn, DataTable i_dSourceTable)
{
    DataView dv = new DataView(i_dSourceTable);
    //getting distinct values for group column
    DataTable dtGroup = dv.ToTable(true, new string[] { i_sGroupByColumn });
    //adding column for the row count
    dtGroup.Columns.Add("Count", typeof(int));
    //looping thru distinct values for the group, counting
    foreach (DataRow dr in dtGroup.Rows) {
        dr["Count"] = i_dSourceTable.Compute("Count(" + i_sAggregateColumn + ")", i_sGroupByColumn + " = '" + dr[i_sGroupByColumn] + "'");
    }
    //returning grouped/counted result
    return dtGroup;
}

例：

DataTable desiredResult = GroupBy("TeamID", "MemberID", dt);

在 LINQ 中分组依据 

var listInfo = (from infoMember in context.Members
                where infoMember.TeamID  == TeamID 
                group infoMember by new
                { infoMember.TeamID, infoMember.MemberIDCount } into newInfoMemeber
                select new ClassName
                {
                   TeamID = newInfo.Key.TeamID,
                   MemberIDCount = newInfo.Key.MemberIDCOunt,
                   Count = newInfo.Count(),
                   TotalCount = (from infoMemeber2 in context.Members
                                 where infoMemeber2.TeamID== TeamID
                                 select infoResult2).Count()
                }).AsEnumerable();
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