高效的DataTable Group By

Question

我想在DataTable上执行聚合查询以创建另一个DataTable.我无法改变用于创建初始DataTable的SQL.

原始DataTable :(一切都是int)

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

期望的结果:

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

如果它是SQL我可以做

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

但在我的应用程序中,我知道如何处理这个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);
    }
}

有没有更好的办法？

Answer 1

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);

Answer 2

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);