Truy vấn 1 giá trị ta dùng hàm ExecuteScalar trong c#

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 kncsdlhocba

{

    public partial class Truyvandulieudondong : Form

    {

        public Truyvandulieudondong()

        {

            InitializeComponent();

        }

        SqlConnection conec = null;

        string ketnoi = @”Server =DESKTOP-CR8PM8T\SQLEXPRESS; Database=CSDLSANPHAM ; User ID=sa; pwd =123″;

        private void btnXem_Click(object sender, EventArgs e)

        {

            if (conec==null)

            {

                conec = new SqlConnection(ketnoi);

            }

            if (conec.State == ConnectionState.Closed)

            {

                conec.Open();

            }  

            SqlCommand command = new SqlCommand();

            command.CommandType = CommandType.Text;

            command.CommandText = “select count (*) from SanPham”;

            command.Connection = conec;

            object data = command.ExecuteScalar();// chắc chắn trả về 1 giá trị ta dùng ExecuteScalar

            int n = (int)data;// chắc chắn nó là kiểu int

            btnXem.Text = “có ” + n + ” sản phẩm”           

        }

Truy vấn 1 giá trị ta dùng hàm ExecuteReader

public partial class Truyvandulieudondong : Form

    {

        public Truyvandulieudondong()

        {

            InitializeComponent();

        }

        SqlConnection conec = null;

        string ketnoi = @”Server =DESKTOP-CR8PM8T\SQLEXPRESS; Database=CSDLSANPHAM ; User ID=sa; pwd =123″;

        private void btnXem_Click(object sender, EventArgs e)

        {

            if (conec==null)

            {

                conec = new SqlConnection(ketnoi);

            }

            if (conec.State == ConnectionState.Closed)

            {

                conec.Open();

            }

            SqlCommand command = new SqlCommand();

            command.CommandType = CommandType.Text;

            command.CommandText = “select * from SanPham where ma=” + txtMamuontim.Text;

            command.Connection = conec;

            SqlDataReader reader = command.ExecuteReader();

            if (reader.Read())

            {

                txtMa.Text = reader.GetInt32(0) + “”;

                txtTen.Text = reader.GetString(1);

                txtGia.Text = reader.GetInt32(2).ToString();

            }

            reader.Close();

Truy vấn 1 giá trị ta dùng hàm ExecuteReader với prameter

private void txtpara_Click(object sender, EventArgs e)

        {

            if (conec==null)

            {

                conec = new SqlConnection(ketnoi);

            }

            if (conec.State== ConnectionState.Closed)

            {

                conec.Open();

            }

            SqlCommand command = new SqlCommand();

            command.CommandType = CommandType.Text;

            command.CommandText = ” select * from SanPham where ma=@ma”;

            command.Connection = conec;

            SqlParameter parama = new SqlParameter(“@ma”, SqlDbType.Int);

            parama.Value = txtMamuontim.Text;

            command.Parameters.Add(parama);

            SqlDataReader reader = command.ExecuteReader();

            if (reader.Read())

            {

                txtMa.Text = reader.GetInt32(0) + “”;

                txtTen.Text = reader.GetString(1);

                txtGia.Text = reader.GetInt32(2) + “”;

            }

            reader.Close();

        }