Nested If condition

You can match the both dropdown selected value and use common method to call saving method logic depending on dropdown selected. Make one common method and in common method pass the statement and parameter correct depending on related matched value of dropdown. Just check the sample for your understanding for your reference. Call save method depending on Dropdown Selected. Below code is just for your reference you can use by your own way by your programing logic just refer how to match and call save method.

Save(DropDownList1.SelectedItem.Value); // If Dropdown1 selected call this statement
Save(DropDownList2.SelectedItem.Value); // If Dropdown2 selected call this statement

  Create Save method

private void Save(string ddlValue)
{
    // All Connection Declaration which you are using for sql    
    string conString = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
    SqlConnection con = new SqlConnection(conString);
    SqlCommand cmd;

    // Then match the condition depending on value selected for dropdown
    if (ddlValue == "Residential")
    {
        /* Your code for Residential with related sql stament and parameter as i added for Non residential */
    }
    else if (ddlValue == "Non Residential")
    {
        cmd = new SqlCommand("insert into Cal_NonR(Title_of_Programme,GroupID,Program_Start_Date,Program_End_Date,Venue,Fee,PO_Number) values(@Title_of_Programme,@GroupID,@Program_Start_Date,@Program_End_Date,@Venue,@Fee,@PO_Number)", con);
        cmd.Parameters.AddWithValue("@Title_of_Programme", TextBox1.Text);
        cmd.Parameters.AddWithValue("@GroupID", DropDownList2.SelectedItem.Text);
        cmd.Parameters.AddWithValue("@Program_Start_Date", Convert.ToDateTime(TextBox5.Text));
        cmd.Parameters.AddWithValue("@Program_End_Date", Convert.ToDateTime(TextBox6.Text));
        cmd.Parameters.AddWithValue("@Venue", tbVenue.Text);
        cmd.Parameters.AddWithValue("@Fee", tbTotalFee.Text);
        cmd.Parameters.AddWithValue("@PO_Number", TextBox4.Text);
    }
    else if (ddlValue == "HES01")
    {
        cmd = new SqlCommand("insert into HES01(GroupID,ProgrammeType,Title,ProgramStartDate ,ProgramEndDate,FiancialYear,Venue,TFee,PO_number) values(@GroupID,@ProgrammeType,@Title,@ProgramStartDate,@ProgramEndDate,@FiancialYear,@Venue,@TFee,@PO_number)", con);
        cmd.Parameters.AddWithValue("@GroupID", DropDownList2.SelectedItem.Text);
        cmd.Parameters.AddWithValue("@ProgrammeType", DropDownList1.SelectedItem.Text);
        cmd.Parameters.AddWithValue("@Title", TextBox1.Text);
        cmd.Parameters.AddWithValue("@ProgramStartDate", Convert.ToDateTime(TextBox5.Text));
        cmd.Parameters.AddWithValue("@ProgramEndDate", Convert.ToDateTime(TextBox6.Text));
        cmd.Parameters.AddWithValue("@FiancialYear", TextBox7.Text);
        cmd.Parameters.AddWithValue("@Venue", tbVenue.Text);
        cmd.Parameters.AddWithValue("@TFee", tbTotalFee.Text);
        cmd.Parameters.AddWithValue("@PO_number", TextBox4.Text);
    }
    else if (ddlValue == "HAG02")
    {
        /* Your code for HAG02 with related sql stament and parameter as i added for Non HES01 */
    }

    /* Then Open connection and Execute the statement then close connection */
    con.Open();
    cmd.ExecuteNonQuery();
    con.Close();
}