如何在DataTable.Select(Expression)中使用SELECT GROUP BY？

Question

我尝试通过从每个组中选择第一行来删除重复的行.例如

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

我想要一个回报:

PK     Col1     Col2
1        A        B
3        C        C

我尝试了下面的代码,但它不起作用:

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

Answer 1

DataTable的Select方法只支持简单的过滤表达式,如{field} = {value}.它不支持复杂的表达式,更不用说SQL/Linq语句了.

但是,您可以使用Linq扩展方法提取DataRows 的集合,然后创建新的 DataTable.

dt = dt.AsEnumerable()
       .GroupBy(r => new {Col1 = r["Col1"], Col2 = r["Col2"]})
       .Select(g => g.OrderBy(r => r["PK"]).First())
       .CopyToDataTable();

Answer 2

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();

Answer 3

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

Answer 4

蒂姆·施密特的答案/sf/answers/593043111/

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);