Question

我想创建类似if (reader["assignID"].count > 2)的东西，然后发生一些事情。但我知道MySqlDataReader没有计数功能。我还应该如何处理呢？

我要计算行数。

我的代码

try
{
    string myConnectionString3;
    myConnectionString3 = "server=localhost;uid=root;pwd=root;database=medicloud;SslMode=None;charset=utf8";

    MySqlConnection connection3 = new MySqlConnection(myConnectionString3);
    MySqlCommand cmd3 = new MySqlCommand();

    EncodingProvider ppp3;
    ppp3 = CodePagesEncodingProvider.Instance;
    Encoding.RegisterProvider(ppp3);

    cmd3.CommandType = CommandType.Text;
    string sqlStr2 = "Select assignID from assign where userId=@name";
    cmd3.Parameters.AddWithValue("@name", txtValue.Text);
    cmd3.CommandText = sqlStr2;
    cmd3.Connection = connection3;


    connection3.Open();
    MySqlDataReader reader1 = cmd3.ExecuteReader();


    if (reader1.HasRows)
    {
        while (reader1.Read())
        {
            if (reader1["assignID"].count > 2) //count rows if more than 2
            {
                txtAssignID1.Text += "Hello";
            }
            else
            {
                btnStart.IsEnabled = true;
                string assignID = (reader1["assignID"].ToString());
                txtAssignID1.Text += "Your exercise ID is: " + assignID;
            }
        }                                                                                             
    }
    else
    {

        txtAssignID1.Text += "You have not been assigned any exercise ID";
        txtAssignID.IsEnabled = false;
    }

    connection3.Close();
}
catch (MySqlException)
{

}

Answer 1

如果您想知道单个用户有多少个 assignID ，则应将其询问给数据库。将查询更改为

string sqlStr2 = "Select COUNT(assignID) as Totals from assign where userId=@name";

然后使用 ExecuteScalar 取回结果

 int assignCount = 0;
 // We need to check for nulls, because if the where condition doesn't 
 // match any userid then ExecuteScalar returns null.
 object result = cmd3.ExecuteScalar();
 if(result != null)
    assignCount = Convert.ToInt32(result);

并删除所有数据读取器的内容。

如果您还需要知道AssignID的值，则应返回MySqlDataReader方法，但再次使用不同的查询

string sqlStr2 = @"Select assignID, COUNT(assignID) as Totals 
                  from assign where userId=@name
                  GROUP BY assignID";

.....
MySqlDataReader reader1 = cmd3.ExecuteReader();
if (reader1.HasRows)
{
    while (reader1.Read())
    {
        if ((int)reader1["Totals"] > 2) 
        {
            // Not clear what you want to do if Totals > 2
            txtAssignID1.Text += "Hello";
        }
        else
        {
            btnStart.IsEnabled = true;
            string assignID = (reader1["assignID"].ToString());
            txtAssignID1.Text += "Your exercise ID is: " + assignID;
        }
    }  
}
else
{
    txtAssignID1.Text += "You have not been assigned any exercise ID";
    txtAssignID.IsEnabled = false;
}

Answer 2

如果只需要知道计数是否大于2，则最好使用计数查询并执行GetScalar。这样，您仅检索必要的数据。

数据库c＃if（reader [“”]。count> 2）

2 个答案: