Connection.Open();
string dd = DateTime.Now.ToShortDateString();
string qry = "UPDATE TAccount SET Amount = Amount +" +
              a.transactionAmount.ToString() +
              " WHERE ID = " + a.ID.ToString() +
          " insert into TransactionHistory (ID, CardNumber, Amount, TransactionDate)
        values ((SELECT MAX(ID) FROM TransactionHistory)+1, " +
           a.ID.ToString() + " , " + a.transactionAmount.ToString() +
           " , " + dd.ToString() + ")";
SqlCommand cmd = new SqlCommand(qry, Connection);

cmd.ExecuteNonQuery();
Connection.Close();