How to merge multiple datatables (every datatable return one row) in one row

Question

I have multiple data tables : i want to merge them (My P.K : emp_num)

var tblEmp_data = new DataTable();//one row
var tblEmp_time = new DataTable();//one row
var tbl_emp_mission = new DataTable();//one row

tblEmp_data = GetEmpInfo(empNum);
tblEmp_time = GetEmpTime(empNum, start_period, end_period);
tbl_emp_mission = GetEmpMission(empNum, start_period, end_period);

tblEmp_data.Merge(tblEmp_time, false, MissingSchemaAction.Add);
tblEmp_data.AcceptChanges();
tblEmp_data.Merge(tbl_emp_mission, false, MissingSchemaAction.Add);
tblEmp_data.AcceptChanges();

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Now i get the data on multiple rows rather than one row ! I want the data on one row ? how to do this ?

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Note: i want all the columns allow null except the primary key so i avoid this exception :

failed to enable constraints. one or more rows contain values violating non-null, unique, or foreign-key constraints 

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Edited:

The third table which cause the problem:

public static DataTable GetEmpMission(int empNum, DateTime start_period, DateTime end_period)
        {
            using (IfxConnection con = new IfxConnection(ConfigurationManager.ConnectionStrings["tl"].ToString()))
            {
                DataTable dt = new DataTable();
                StringBuilder cmdTxt = new StringBuilder();
                cmdTxt.Append(" SELECT COUNT(emp_num)  ");
                cmdTxt.Append(" FROM hs_mission WHERE emp_num = ? AND from_date BETWEEN ? AND ?  ");

                using (var myIfxCmd = new IfxCommand(cmdTxt.ToString(), con))
                {

                    myIfxCmd.CommandType = CommandType.Text;
                    if (con.State == ConnectionState.Closed)
                    {
                        con.Open();
                    }
                    myIfxCmd.Parameters.Add("emp_num", IfxType.SmallInt);
                    myIfxCmd.Parameters.Add("start_period", IfxType.Date);
                    myIfxCmd.Parameters.Add("end_period", IfxType.Date);

                    myIfxCmd.Parameters[0].Value = ((object)empNum) ?? DBNull.Value;
                    myIfxCmd.Parameters[1].Value = ((object)start_period.Date) ?? DBNull.Value;
                    myIfxCmd.Parameters[2].Value = ((object)end_period.Date) ?? DBNull.Value;

                    using (IfxDataReader dr = myIfxCmd.ExecuteReader())
                    {
                        dt.Load(dr);
                        dt.Columns.Add("emp_num", typeof(Int32));
                        dt.Rows[0]["emp_num"] = empNum;
                        dt.AcceptChanges();
                    }

                }
                con.Close();
                con.Dispose();
                return dt;


            }

        }

---

Return data like this :

column1            emp_num 

 0                   6762

and throw exception :

failed to enable constraints. one or more rows contain values violating non-null, unique, or foreign-key constraints 

Answer 1

So empNum is the key column which all tables share? It sounds as if you could use my MergeAll:

public static DataTable MergeAll(this IList<DataTable> tables, String primaryKeyColumn)
{
    if (!tables.Any())
        throw new ArgumentException("Tables must not be empty", "tables");
    if(primaryKeyColumn != null)
        foreach(DataTable t in tables)
            if(!t.Columns.Contains(primaryKeyColumn))
                throw new ArgumentException("All tables must have the specified primarykey column " + primaryKeyColumn, "primaryKeyColumn");

    if(tables.Count == 1)
        return tables[0];

    DataTable table = new DataTable("TblUnion");
    table.BeginLoadData(); // Turns off notifications, index maintenance, and constraints while loading data
    foreach (DataTable t in tables)
    {
        foreach (DataColumn col in t.Columns) 
            col.ReadOnly = false; // this might be required in your case
        table.Merge(t); // same as table.Merge(t, false, MissingSchemaAction.Add);
    }
    table.EndLoadData();

    if (primaryKeyColumn != null)
    {
        // since we might have no real primary keys defined, the rows now might have repeating fields
        // so now we're going to "join" these rows ...
        var pkGroups = table.AsEnumerable()
            .GroupBy(r => r[primaryKeyColumn]);
        var dupGroups = pkGroups.Where(g => g.Count() > 1);
        foreach (var grpDup in dupGroups)
        { 
            // use first row and modify it
            DataRow firstRow = grpDup.First();
            foreach (DataColumn c in table.Columns)
            {
                if (firstRow.IsNull(c))
                {
                    DataRow firstNotNullRow = grpDup.Skip(1).FirstOrDefault(r => !r.IsNull(c));
                    if (firstNotNullRow != null)
                        firstRow[c] = firstNotNullRow[c];
                }
            }
            // remove all but first row
            var rowsToRemove = grpDup.Skip(1);
            foreach(DataRow rowToRemove in rowsToRemove)
                table.Rows.Remove(rowToRemove);
        }
    }

    return table;
}

Then use it in this way:

var tables = new[] { tblEmp_data, tblEmp_time, tbl_emp_mission };
tblEmp_data = tables.MergeAll("empNum");