[c#] How do I use SELECT GROUP BY in DataTable.Select(Expression)?

I try to remove the duplicate rows by select a first row from every group. For Example

PK     Col1     Col2
1        A        B
2        A        B
3        C        C
4        C        C

I want a return:

PK     Col1     Col2
1        A        B
3        C        C

I tried following code but it didn't work:

DataTable dt = GetSampleDataTable(); //Get the table above.
dt = dt.Select("SELECT MIN(PK), Col1, Col2 GROUP BY Col1, Col2);

dt = dt.AsEnumerable().GroupBy(r => r.Field<int>("ID")).Select(g => g.First()).CopyToDataTable();

dt.AsEnumerable()
    .GroupBy(r => new { Col1 = r["Col1"], Col2 = r["Col2"] })
    .Select(g =>
    {
        var row = dt.NewRow();

        row["PK"] = g.Min(r => r.Field<int>("PK"));
        row["Col1"] = g.Key.Col1;
        row["Col2"] = g.Key.Col2;

        return row;

    })
    .CopyToDataTable();

This solution sort by Col1 and group by Col2. Then extract value of Col2 and display it in a mbox.

var grouped = from DataRow dr in dt.Rows orderby dr["Col1"] group dr by dr["Col2"];
string x = "";
foreach (var k in grouped) x += (string)(k.ElementAt(0)["Col2"]) + Environment.NewLine;
MessageBox.Show(x);