comparing the tow tables and not matching items to be displayed in the datagridview column

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C#

i have one combo box and View(Button)

comparing two tables and not matching items to be displayed in the datagridview column.

i compare two tables in combo box values are selected and when i click the view button not matching items from the combo box will be displayed in the datagridview column.

private void  View_Click(object sender, EventArgs e)
        {
             sql = "select Faculty_Code from Tb_Selected_Faculty_Details where Faculty_Code = '" + Cb_Faculty_Code.Text + "'";
             oledr = GFun.ReadAcessSql(sql);
             oledr.Read();
             if (oledr.HasRows == true)
             {
                 MessageBox.Show("Selected Faculty is already allocated");
                 return;
                 else
                 {
                    DataTable dt = new DataTable();
                    dt.Columns.Add(Cb_Faculty_Code.Text);
                 dt.Rows.Add(new object[] {Cb_Faculty_Code.SelectedItem.ToString()});
                    datagridView.DataSource = dt;
                 }
             }
        }

whether the view button code is correct.non matching items from the combo box will be displayed in the data gri dveiw column.

how can i do> view button code is correct.please help me.