c# - 需要写一个总和(成绩)/总和(学分)代码

Question

我正在制作一个学生数据库，其中包含类(class) ID、类(class)名称、每门类(class)的学分、通过/失败声明和成绩。我的目标是让按钮显示总和(成绩)/总和(学分)

这是我编写的代码，它给我一个错误，说“已经有一个与此连接关联的打开的数据读取器，必须首先关闭”。这是我的代码:

private void button1_Click(object sender, EventArgs e)
        {
           string ConString = " datasource = localhost; port = 3306; username = root; password = 3306";
           string Query = " Select sum(grade) from studentdata.semestre1";
           string Query1 = " Select sum(grade) from studentdata.semestre1";
           MySqlConnection ConDatabase = new MySqlConnection(ConString);
           MySqlCommand cmdDataBase = new MySqlCommand(Query, ConDatabase);
           MySqlCommand cmdDataBase1 = new MySqlCommand(Query1, ConDatabase);

           MySqlDataReader myReader;
           MySqlDataReader myReader1;

            ConDatabase.Open();

            myReader = cmdDataBase.ExecuteReader() ;
            myReader1 = cmdDataBase1.ExecuteReader();

            while ((myReader.Read()) && (myReader1.Read()))
            {
                textBox2.Text =Convert.ToString(double.Parse( myReader.GetString(0))/ double.Parse(myReader1.GetString(0))) ;
            }
            myReader.Close();
            myReader1.Close();

有人可以帮忙吗？

Answer 1

该错误表明数据读取器在其他位置处于打开状态。它类似于连接对象。您不必创建两次，您可以在单个查询中执行相同的操作

  string ConString = " datasource = localhost; port = 3306; username = root; password = 3306";
  string Query = "Select SUM(grade)/SUM(credits) FROM studentdata.semestre1";
  MySqlConnection ConDatabase = new MySqlConnection(ConString);
  MySqlCommand cmdDataBase = new MySqlCommand(Query, ConDatabase);
  MySqlDataReader myReader;
  ConDatabase.Open();

  while ((myReader.Read()) && (myReader1.Read()))
        {
            textBox2.Text =Convert.ToString(double.Parse( myReader.GetString(0))) ;
        }
  myReader.Close();