using MySql.Data.MySqlClient;
String connetStr = "server=127.0.0.1;port=3306;user=root;password=root; database=minecraftdb;";
// server=127.0.0.1/localhost 代表本機(jī)，端口號(hào)port默認(rèn)是3306可以不寫(xiě)
MySqlConnection conn = new MySqlConnection(connetStr);
try
{  
   conn.Open();//打開(kāi)通道，建立連接，可能出現(xiàn)異常,使用try catch語(yǔ)句
   Console.WriteLine("已經(jīng)建立連接");
   //在這里使用代碼對(duì)數(shù)據(jù)庫(kù)進(jìn)行增刪查改
}
catch (MySqlException ex)
{
   Console.WriteLine(ex.Message);
}
finally
{
   conn.Close();
}

catch (MySqlException ex)
{
  switch (ex.Number)
  {
    case 0:
    Console.WriteLine("Cannot connect to server. Contact administrator");
    break;
  case 1045:
    Console.WriteLine("Invalid username/password, please try again");
    break;
  }
}

string sql= "select * from user";
MySqlCommand cmd = new MySqlCommand(sql,conn);
MySqlDataReader reader =cmd.ExecuteReader();//執(zhí)行ExecuteReader()返回一個(gè)MySqlDataReader對(duì)象
while (reader.Read())//初始索引是-1，執(zhí)行讀取下一行數(shù)據(jù)，返回值是bool
{
  //Console.WriteLine(reader[0].ToString() + reader[1].ToString() + reader[2].ToString());
  //Console.WriteLine(reader.GetInt32(0)+reader.GetString(1)+reader.GetString(2));
  Console.WriteLine(reader.GetInt32("userid") + reader.GetString("username") + reader.GetString("password"));//"userid"是數(shù)據(jù)庫(kù)對(duì)應(yīng)的列名，推薦這種方式
}

//string sql = "select * from user where username='"+username+"' and password='"+password+"'"; //我們自己按照查詢條件去組拼
string sql = "select * from user where username=@para1 and password=@para2";//在sql語(yǔ)句中定義parameter，然后再給parameter賦值
MySqlCommand cmd = new MySqlCommand(sql, conn);
cmd.Parameters.AddWithValue("para1", username);
cmd.Parameters.AddWithValue("para2", password);

MySqlDataReader reader = cmd.ExecuteReader();
if (reader.Read())//如果用戶名和密碼正確則能查詢到一條語(yǔ)句，即讀取下一行返回true
{
  return true;
}

string sql = "select count(*) from user";
MySqlCommand cmd = new MySqlCommand(sql, conn);
Object result=cmd.ExecuteScalar();//執(zhí)行查詢，并返回查詢結(jié)果集中第一行的第一列。所有其他的列和行將被忽略。select語(yǔ)句無(wú)記錄返回時(shí)，ExecuteScalar()返回NULL值
if (result != null)
{
  int count = int.Parse(result.ToString());
}

string sql = "insert into user(username,password,registerdate) values('啊寬','123','"+DateTime.Now+"')";
//string sql = "delete from user where userid='9'";
//string sql = "update user set username='啊哈',password='123' where userid='8'";
MySqlCommand cmd = new MySqlCommand(sql,conn);
int result =cmd.ExecuteNonQuery();//3.執(zhí)行插入、刪除、更改語(yǔ)句。執(zhí)行成功返回受影響的數(shù)據(jù)的行數(shù)，返回1可做true判斷。執(zhí)行失敗不返回任何數(shù)據(jù)，報(bào)錯(cuò)，下面代碼都不執(zhí)行

String connetStr = "server=127.0.0.1;user=root;password=root;database=minecraftdb;";
MySqlConnection conn = new MySqlConnection(connetStr);
conn.Open();//必須打開(kāi)通道之后才能開(kāi)始事務(wù)
MySqlTransaction transaction = conn.BeginTransaction();//事務(wù)必須在try外面賦值不然catch里的transaction會(huì)報(bào)錯(cuò):未賦值
Console.WriteLine("已經(jīng)建立連接");
try
{
  string date = DateTime.Now.Year + "-" + DateTime.Now.Month + "-" + DateTime.Now.Day;
  string sql1= "insert into user(username,password,registerdate) values('啊寬','123','" + date + "')";
  MySqlCommand cmd1 = new MySqlCommand(sql1,conn);
  cmd1.ExecuteNonQuery();

  string sql2 = "insert into user(username,password,registerdate) values('啊寬','123','" + date + "')";
  MySqlCommand cmd2 = new MySqlCommand(sql2, conn);
  cmd2.ExecuteNonQuery();
}
catch (MySqlException ex)
{
  Console.WriteLine(ex.Message);
  transaction.Rollback();//事務(wù)ExecuteNonQuery()執(zhí)行失敗報(bào)錯(cuò)，username被設(shè)置unique
  conn.Close();
}
finally
{
  if (conn.State != ConnectionState.Closed)
  {
    transaction.Commit();//事務(wù)要么回滾要么提交，即Rollback()與Commit()只能執(zhí)行一個(gè)
    conn.Close();
  }
}