高效的 DataTable 分组依据

Efficient DataTable Group By(高效的 DataTable 分组依据)

问题描述

I would like to perform an aggregate query on a DataTable to create another DataTable. I cannot alter the SQL being used to create the initial DataTable.

Original DataTable: (everything is an int)

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

Desired result:

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

If it were SQL I could just do

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

but in my application, the only way I know how to handle this would be something like this:

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);
    }
}

Is there a better way?

You may use 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);

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