using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.OleDb;
using System.Windows.Forms;
using System.Data;

namespace Test
{
    /// <summary>
    /// 資料庫操作類
    /// Data 2011-4-20
    /// Author Laiminghua
    /// </summary>
    public static class DBHelp
    {
        #region 變數宣告處
        private static OleDbConnection Conn;
        private static string ConnString = "Provider=Microsoft.Jet.OleDb.4.0;Data Source=" + Application.StartupPath + @"/";//連線字串
        #endregion

        #region 建構函式與連線關閉資料庫

        /// <summary>
        /// 開啟資料來源連結
        /// </summary>
        /// <returns></returns>
        public static void OpenConn()
        {
            Conn = new OleDbConnection(ConnString);
            Conn.Open();
        }

        /// <summary>
        /// 請在資料傳遞完畢後呼叫該函式，關閉資料鏈接。
        /// </summary>
        public static void CloseConn()
        {
            Conn.Close();
        }
        #endregion

        #region 資料庫基本操作
        /// <summary>
        /// 根據SQL命令返回資料DataTable資料表,
        /// 可直接作為dataGridView的資料來源
        /// </summary>
        /// <param name="SQL"></param>
        /// <returns></returns>
        public static DataTable SelectToDataTable(string SQL)
        {
            OleDbDataAdapter adapter = new OleDbDataAdapter();
            OleDbCommand command = new OleDbCommand(SQL, Conn);
            adapter.SelectCommand = command;
            DataTable Dt = new DataTable();
            adapter.Fill(Dt);   
            return Dt;
        }

        /// <summary>
        /// 根據SQL命令返回資料DataSet資料集，其中的表可直接作為dataGridView的資料來源。
        /// </summary>
        /// <param name="SQL"></param>
        /// <param name="subtableName">在返回的資料集中所新增的表的名稱</param>
        /// <returns></returns>
        public static DataSet SelectToDataSet(string SQL, string subtableName)
        {
            OleDbDataAdapter adapter = new OleDbDataAdapter();
            OleDbCommand command = new OleDbCommand(SQL, Conn);
            adapter.SelectCommand = command;
            DataSet Ds = new DataSet();
            Ds.Tables.Add(subtableName);
            adapter.Fill(Ds, subtableName);
            return Ds;
        }

        /// <summary>
        /// 在指定的資料集中新增帶有指定名稱的表，由於存在覆蓋已有名稱表的危險，返回操作之前的資料集。
        /// </summary>
        /// <param name="SQL"></param>
        /// <param name="subtableName">新增的表名</param>
        /// <param name="DataSetName">被新增的資料集名</param>
        /// <returns></returns>
        public static DataSet SelectToDataSet(string SQL, string subtableName, DataSet DataSetName)
        {
            OleDbDataAdapter adapter = new OleDbDataAdapter();
            OleDbCommand command = new OleDbCommand(SQL, Conn);
            adapter.SelectCommand = command;
            DataTable Dt = new DataTable();
            DataSet Ds = new DataSet();
            Ds = DataSetName;
            adapter.Fill(DataSetName, subtableName);
            return Ds;
        }

        /// <summary>
        /// 根據SQL命令返回OleDbDataAdapter，
        /// 使用前請在主程式中新增名稱空間System.Data.OleDb
        /// </summary>
        /// <param name="SQL"></param>
        /// <returns></returns>
        public static OleDbDataAdapter SelectToOleDbDataAdapter(string SQL)
        {
            OleDbDataAdapter adapter = new OleDbDataAdapter();
            OleDbCommand command = new OleDbCommand(SQL, Conn);
            adapter.SelectCommand = command;
            return adapter;
        }

        /// <summary>
        /// 執行刪除
        /// </summary>
        /// <param name="SQL"></param>
        /// <returns></returns>
        public static bool ExecuteSQLNonquery(string SQL)
        {
            OleDbCommand cmd = new OleDbCommand(SQL, Conn);
            try
            {
                cmd.ExecuteNonQuery();
                return true;
            }
            catch
            {
              return false;
            }
        }


        /// <summary>
        /// 執行增，改
        /// </summary>
        /// <param name="SQL"></param>
        /// <returns></returns>
        public static int ExecuteSQLScalar(string SQL)
        {
            OleDbCommand cmd = new OleDbCommand(SQL, Conn);
            try
            {
               int result= Convert.ToInt32(cmd.ExecuteScalar());
               return result;
            }
            catch
            {
                return 0;
            }
        }
        #endregion
    }
}

呼叫

//查詢
            DBHelp.OpenConn();
            DataTable dt = DBHelp.SelectToDataTable("select * from table");
            for
            DBHelp.CloseConn();