sql foem

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using Npgsql;

namespace psqlTest
{
    public partial class Form2 : Form
    {
        string host = global.host;
        string database = global.database;
        string user = global.user;
        string password = global.password;

        private string connstring = string.Format("Server={0};Port={1};" +
            "User Id = {2};Password={3};Database={4};",
            global.host, global.port, global.user,
            global.password, global.database);
        private NpgsqlConnection conn;
        private string sql;
        private NpgsqlCommand cmd;
        private DataTable dt;
        private int rowIndex = -1;
        public Form2()
        {
            InitializeComponent();
        }

        private void Form2_Load(object sender, EventArgs e)
        {
            conn = new NpgsqlConnection(connstring);
            Select();
        }

        private void Select()
        {
            try
            {
                conn.Open();
                sql = @"select * from st_select();";
                cmd = new NpgsqlCommand(sql, conn);
                dt = new DataTable();
                dt.Load(cmd.ExecuteReader());
                conn.Close();
                dvgData.DataSource = null;
                dvgData.DataSource = dt;
            }
            catch (Exception ex)
            {
                conn.Close();
                MessageBox.Show("Error: "+ex.Message);

                throw;
            }
        }

        private void dvgData_CellClick(object sender, DataGridViewCellEventArgs e)
        {
            if (e.RowIndex >= 0)
            {
                rowIndex = e.RowIndex;
                txtFirst.Text = dvgData.Rows[e.RowIndex].Cells["firstname"].Value.ToString();
                txtMiddle.Text = dvgData.Rows[e.RowIndex].Cells["midname"].Value.ToString();
                txtLast.Text = dvgData.Rows[e.RowIndex].Cells["lastname"].Value.ToString();
            }
        }

        private void btnIncert_Click(object sender, EventArgs e)
        {
            txtFirst.Enabled = txtMiddle.Enabled = txtLast.Enabled = true;
            rowIndex = -1;
            txtFirst.Text = txtMiddle.Text = txtLast.Text = null;
            txtFirst.Select();
        }

        private void btnDelete_Click(object sender, EventArgs e)
        {
            if (rowIndex < 0)
            {
                MessageBox.Show("Please choose student to delete");
                return;
            }
            try
            {
                conn.Open();
                sql = @"select * from st_delete(:_id)";
                cmd = new NpgsqlCommand(sql, conn);
                cmd.Parameters.AddWithValue("_id", int.Parse(dvgData.Rows[rowIndex].Cells["id"].Value.ToString()));
                if ((int)cmd.ExecuteScalar() == 1)
                {
                    MessageBox.Show("Delete student successfully");
                    rowIndex = -1;
                    Select();
                }
                conn.Close();
            }
            catch (Exception ex)
            {
                conn.Close();
                MessageBox.Show("Deleted fail. Error:"+ex.Message);
            }
        }

        private void btnUpdate_Click(object sender, EventArgs e)
        {
            if(rowIndex < 0)
            {
                MessageBox.Show("Please choose student to update");
                return;
            }
            txtFirst.Enabled = txtMiddle.Enabled = txtLast.Enabled = true;
        }

        private void btnSave_Click(object sender, EventArgs e)
        {
            int result = 0;
            if (rowIndex < 0)
            {
                try
                {
                    conn.Open();
                    sql = @"select * from st_insert(:_firstname, :_midname, :_lastname)";
                    cmd = new NpgsqlCommand(sql, conn);
                    cmd.Parameters.AddWithValue("_firstname",txtFirst.Text);
                    cmd.Parameters.AddWithValue("_midname", txtMiddle.Text);
                    cmd.Parameters.AddWithValue("_lastname", txtLast.Text);
                    result = (int)cmd.ExecuteScalar();
                    conn.Close();
                    if(result == 1)
                    {
                        MessageBox.Show("Inserted new student successfully.");
                        Select();
                    }
                    else
                    {
                        MessageBox.Show("Inserted fall.");
                    }
                }
                catch (Exception ex)
                {
                    MessageBox.Show("Insert fail. Error:"+ex.Message);
                    conn.Close();
                }
            }
            else
            {
                try
                {
                    conn.Open();
                    sql = @"select * from st_update(:_id,:_firstname,:_midname,:_lastname)";
                    cmd = new NpgsqlCommand(sql, conn);
                    cmd.Parameters.AddWithValue("_id",int.Parse(dvgData.Rows[rowIndex].Cells["id"].Value.ToString())); ;
                    cmd.Parameters.AddWithValue("_firstname", txtFirst.Text);
                    cmd.Parameters.AddWithValue("_midname", txtMiddle.Text);
                    cmd.Parameters.AddWithValue("_lastname", txtLast.Text);
                    result = (int)cmd.ExecuteScalar();
                    conn.Close();
                    if (result == 1)
                    {
                        MessageBox.Show("Updated student successfully.");
                        Select();
                    }
                    else
                    {
                        MessageBox.Show("Update failed.");
                    }
                }

                catch (Exception ex)
                {
                    MessageBox.Show("Updated fail. Error:" + ex.Message);
                    conn.Close();
                }

            }
            result = 0;
            txtFirst.Text = txtMiddle.Text = txtLast.Text = null;
            txtFirst.Enabled = txtMiddle.Enabled = txtLast.Enabled = false;
        }
    }
}