C# fill combobox particular account and their sub accounts so on windows application mysql query

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C#

MySQL

combobox

I created a windows form, with a ComboBox, in C# with Mysql Database, the table name is accounthead from this table I want to populate only particular HeadName and its under all Sub HeadName, example Sales A/C and under all this Sub HeadName only
Mysql Database Table Name: accounthead

HeadCode  HeadName     UnderHead
  1      Cash                0
  2      Bank                0 
  3      Purchase A/C        2
  9      Sales A/C           7
 10      Expense A/C         5
101      Area A              9
102      Area B              9
103      Salesman 1        101
104      Salesman 2        101
105      Salesman 3        102
106      Salesman 4        102
109      Depreciation      202

What I have tried:

void BindHeadName()
{   
    string ConnectionString = Properties.Settings.Default.CurConnStr;
    using (MySqlConnection con = new MySqlConnection(ConnectionString))
    {
        string query = "Select * from accounthead ORDER BY HeadName ASC";
        MySqlCommand cmd = new MySqlCommand(query, con);
        con.Open();
        MySqlDataReader dr = cmd.ExecuteReader();
        while (dr.Read())
        {
            string gname = dr.GetString("HeadName");
            ComboBox.Items.Add(gname);
        }
    }
}