第七章、ADO.Net

using (MySqlConnection conn = new MySqlConnection("Server=localhost;Database=study1;uid=root;pwd=root;Charset=utf8"))
using (MySqlCommand cmd = conn.CreateCommand())
{
    conn.Open();//一定要在执行前Open数据库连接
    cmd.CommandText = "Insert into T_Users(UserName,Password) values(‘中国人‘,‘123‘)";
    int rowCount = cmd.ExecuteNonQuery();
    Console.WriteLine("受影响的行数"+rowCount);
}

cmd.CommandText = "Select count(*) from T_Users";//查询命令：查询T_Users表的总数据条数
long count = (long)cmd.ExecuteScalar();//执行查询命令后获得的结果是一行一列的

using(MySqlConnection conn = new MySqlConnection("Server=localhost;Database=study1;uid=root;pwd=root;Charset=utf8"))
using (MySqlCommand cmd = conn.CreateCommand())
{
    conn.Open();
    //查询命令，查询用户名是amdin的密码
    cmd.CommandText = "select password from T_Users where UserName=‘admin‘";
    //执行查询命令并接收返回的查询命令查询结果
    string pwd = (string)cmd.ExecuteScalar();
    //判断查询结果是否为空，空则没有查询到
    if (string.IsNullOrEmpty(pwd))
    {
        Console.WriteLine("没有找到用户");
    }
    else
    {
        Console.WriteLine("密码是："+pwd);
    }
}
Console.ReadKey();

//创建连接对象
using(MySqlConnection conn = new MySqlConnection("Server=127.0.0.1;Database=study1;uid=root;pwd=root;Charset=utf8"))
using (MySqlCommand cmd = conn.CreateCommand())//创建命令对象
{
    conn.Open();
    cmd.CommandText = "select * from T_Users";
    using (MySqlDataReader reader = cmd.ExecuteReader())
    {
        conn.Close();//Reader读取的时候要保持Connection连接
        while (reader.Read())//Reader的好处是：即使查询数据量大，客户端内存也不会明显增加
        {
            //获得数据库里各个字段的值
            int id = reader.GetInt32("Id");
            string username = reader.GetString("UserName");
            string password = reader.GetString("PassWord");
            Console.WriteLine("Id="+id+";UserName="+username+";PassWord="+password);
        }
    }
}
Console.ReadKey();

static void Main(string[] args)
{
    using (MySqlConnection conn = new MySqlConnection("Server=127.0.0.1;Database=study1;uid=root;pwd=root;Charset=utf8"))
    using (MySqlCommand cmd = conn.CreateCommand())
    {
        conn.Open();
        //把要插入的值用参数传递 @un @pwd  
        cmd.CommandText = "insert into T_Users(UserName,PassWord) Values(@un,@pwd)";
        //填充这个参数
        cmd.Parameters.Add(new MySqlParameter { ParameterName = "@un", Value = "黎庆煌" });
        cmd.Parameters.Add(new MySqlParameter { ParameterName = "@pwd", Value = "123" });
        //执行MySql语句
        int count = cmd.ExecuteNonQuery();
        Console.WriteLine("影响行数：" + count);
    }
    Console.ReadKey();
}

//int i = 0;
//cmd.Parameters.Add(new MySqlParameter("@Age", i));
//cmd.Parameters.Add(new MySqlParameter("@Age", 0));
cmd.Parameters.Add(new MySqlParameter("@Age",(object)0));//可以强制转换这样做避免这个陷阱

using(MySqlConnection conn = new MySqlConnection("Server=127.0.0.1;Database=study1;uid=root;pwd=root;Charset=utf8"))
using (MySqlCommand cmd = conn.CreateCommand())
{
    conn.Open();
    cmd.CommandText = "select * from T_Users";
    using (MySqlDataReader reader = cmd.ExecuteReader())
    {
        while (reader.Read())
        {
            string username = reader.GetString("UserName");
            //string password = reader.GetString("PassWord");
            string password;
            if (reader.IsDBNull(reader.GetOrdinal("PassWord")))
            {
                password = null;
            }
            else
            {
                //GetString/GetInt32无法读取数据库中的null值
                //需要提前用IsDBNull判断
                password = reader.GetString("PassWord");
            }
            int? age;
            if (reader.IsDBNull(reader.GetOrdinal("Age")))
            {
                age = null;
            }
            else
            {
                age = reader.GetInt32("Age");
            }
            //int age = reader.GetInt32("Age");
            Console.WriteLine("用户名："+username+"；密码："+password+"；年龄："+age);
        }
    }
}
Console.ReadKey();

DataSet ds = new DataSet();
MySqlDataAdapter adapter = new MySqlDataAdapter(cmd);
adapter.Fill(ds);
DataTable table = ds.Tables[0];

MySqlTransaction tx = conn.BeginTransaction();//启动事务
try
{
    MySqlHelper.ExecuteNonQuery(conn, "Update t_accounts Set Amount=Amount-1000 where Number=‘0001‘");
    string s = null;
    s.ToLower();
    MySqlHelper.ExecuteNonQuery(conn, "Update t_accounts Set Amount=Amount+1000 where Number=‘0002‘");
    tx.Commit();//执行到这，没有错误则提交事务
}
catch (Exception ex)
{
    tx.Rollback();//如果过程中出现任何错误则执行回滚（当前事务操作全部取消）
}