使用VS2012建立SQL连接和创建SQL命令的正确语法是什么？

Question

I'm not really sure where the SqlConnection , SqlCommand and the Open() / Close() goes. I want to use just the single variable cmd throughout the program, hence not using the SqlCommand cmd = new SqlCommand('SELCT * FROM blabla); format.

EDIT: My code below results to the textbox having the text "System.Data.SqlClient.SqlCommand" when i click the button.

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 System.Data.SqlClient;
using System.Data.SqlTypes;
namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        SqlConnection con = new SqlConnection(@"Data Source=EDIOTH\SQLEXPRESS;
        Initial Catalog=Try; Integrated Security=SSPI");
        SqlCommand cmd = new SqlCommand();


        public Form1()
        {            

            InitializeComponent();

        }

        private void button1_Click(object sender, EventArgs e)
        {
            con.Open();
            cmd.CommandText = "SELECT Pnt_Lname FROM PATIENT WHERE Pnt_ID = 1;";
            txtBox1.Text = cmd.ToString();
            con.Close();
        }
    }
}

Answer 1

you can create constant string to hold the connection string and then you can do as below in your button1_Click

you don't need to call the close method of sql connection when you use using block as below

using(SqlConnection con = new SqlConnection(connectionString))
using(SqlCommand cmd = con.CreateCommand())
{
   cmd.CommandText = "SELECT Pnt_Lname FROM PATIENT WHERE Pnt_ID = 1";
   con.Open();
   txtBox1.Text =cmd.ExecuteScalar() as string;
}

And also if you need to read Pnt_Lname from database you better use ExecuteScalar method

Answer 2

You can use this structure. Use using to properly close and dispose of SqlConnection .

Also, you can define the connection string in your config file and use it from there.

using (SqlConnection conn = new SqlConnection(@"Data Source=EDIOTH\SQLEXPRESS;
    Initial Catalog=Try; Integrated Security=SSPI"))
{
    conn.Open();

    SqlCommand command = conn.CreateCommand();
    command.CommandText = "SELECT Pnt_Lname FROM PATIENT WHERE Pnt_ID = 1";

    txtBox1.Text = (String)command.ExecuteScalar();

}

Answer 3

In case this would be of help to anyone, this is the answer to my question:

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 System.Data.SqlClient;

namespace Book
{
    public partial class frmBook : Form
    {
        SqlConnection con =
            new SqlConnection(@"Data Source=EDIOTH\SQLEXPRESS;
                Initial Catalog=XXDB; Integrated Security=SSPI");
        SqlCommand cmd;
        public frmBook()
        {
            InitializeComponent();
        }

        private void frmBook_Load(object sender, EventArgs e)
        {
            con.Open();
            cmd = new SqlCommand("SELECT min(Book_ID) FROM Book;",con);
            txtID.Text = cmd.ExecuteScalar().ToString();
            cmd = new SqlCommand("SELECT title FROM Book WHERE Book_ID = '" 
                + txtID.Text + "'", con);
            txtTitle.Text = cmd.ExecuteScalar().ToString();
            con.Close();
            btnSave.Enabled = false;
        }

        private void btnNext_Click(object sender, EventArgs e)
        {
            int count = int.Parse(txtID.Text) + 1;
            con.Open();
            cmd = new SqlCommand("SELECT title FROM Book WHERE Book_ID = '"
                + count.ToString() +"'", con);
            txtTitle.Text = cmd.ExecuteScalar().ToString();
            txtID.Text = count.ToString();
            con.Close();
        }

        private void btnNew_Click(object sender, EventArgs e)
        {
            txtID.Text = "";
            txtTitle.Text = "";
            txtAuthor.Text = "";
            btnNew.Enabled = false;
            btnSave.Enabled = true;
        }

        private void btnSave_Click(object sender, EventArgs e)
        {
            con.Open();
            cmd = new SqlCommand("INSERT INTO Book (Book_ID, Title, Author) " +
                "VALUES ('"+ txtID.Text +
                "','"+ txtTitle.Text +
                "','"+ txtAuthor.Text +"');", con);
            cmd.ExecuteNonQuery();
            con.Close();
            MessageBox.Show("Data saved!");
            btnSave.Enabled = false;
        }

        private void btnClose_Click(object sender, EventArgs e)
        {
            this.Close();
        }
    }
}