Linq groupby on datatables

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DataTable1

columnName          colGroup
-----------------   ---------------
OriginCompany	    YES
OriginAddress	    YES
OriginZip	        OK
OriginState	        NO

DataTable2

OriginCompany	OriginAddress	OriginZip   	OriginState
------------    ------------   ------------    ------------      
Company1	    PAdd2_1		    10001	        MI
Company1	    PAdd2_1		    10001	        MI	

Company2	    PAdd2_2		    10002	        MI

Company3	    PAdd2_3		    10003	        MI
Company3	    PAdd2_3		    10003	        MI

What I'm looking for is:
Step 1- From DataTable1, get columnName where colGroup is YES or OK

Step 2- Query DataTable2, and group by on columns returned from DataTable1 (Step1)

What I have tried:

C#

IEnumerable<string> result = 
      (from d in dataTable1.AsEnumerable()
       where
       d.Field<string>("colGroup") == "YES" ||
       d.Field<string>("colGroup") == "OK"
       select d.Field<string>("columnName")).ToList();

       var query = from s in dataTable2.AsEnumerable()
       .GroupBy(s => new { from q in result select q})
       select s;

Posted 14-Dec-21 4:38am

Member 10252017

Updated 15-Dec-21 11:34am

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2 solutions

Solution 1

Maybe not the most efficient, but with few columns, shouldn't make much difference.

asp.net - Remove columns from DataTable in C# - Stack Overflow[^]

So, remove any "not" in the selected list of columns.

Posted 14-Dec-21 9:49am

Gerry Schmitz

Updated 14-Dec-21 9:50am

v2

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George Swan · Accepted Answer · 2021-12-15T11:34:00

It seems to me that working with DataTables is not easy as lots of casting is needed in order to extract anything more meaningful than an object from them. I would do something like this to get the required column names from table1.

C#

//convert to an enumerable of rows
 var rows = table1.AsEnumerable();
 var colNames = rows.Where(r => (string)r["colGroup"] == "YES" || (string)r["colGroup"] == "OK")
     .Select(r => r.Field<string>("columnName"));

Now query table2 accessing the relevant Column by using the Row Indexer that takes a string to index into the Columns.

C#

//instantiate a comparer, see last paragraph
 var comparer= new IEnumerableEqualityComparer<object>();
 var query = table2.AsEnumerable()
   .GroupBy(row =>; colNames.Select(name =>; row[name]), comparer)
     //select a value tuple
   .Select(group =>;(Keys:group.Key, Count :group.Count()));

You can enumerate the query like this

C#

foreach (DataRow row in table2.Rows)
{
    foreach (DataColumn column in table2.Columns)
    {
        Console.WriteLine(row[column]);
    }
}

My generic comparer is too simplistic. Search for IEqualityComparer for SequenceEqual for some better examples

C#

   public class IEnumerableEqualityComparer<T> : IEqualityComparer<IEnumerable<T>>
{
    public bool Equals(IEnumerable<T> x, IEnumerable<T> y)
    {
        return  x.SequenceEqual(y);
    }

    public int GetHashCode(IEnumerable<T> items)
    {
        unchecked//allow overflow
        {
            int hash = 17;
            foreach (var item in items)
            {
                //multiplying by primes improves the distribution
                hash = hash * 31 + item.GetHashCode();
            }
            return hash;
        }
    }
}