ASP.Net C# - 根据条件设置 MySQL 查询和参数

Question

如何根据条件设置 MySQL 查询和参数？

我想要基于“questionSource”的不同查询，如下所示。

但是，在我下面的代码中，'cmd' 在当前上下文中不存在。

或者，我可以为每个条件设置两个不同的函数，并根据需要调用必要的函数，但我想必须有一种方法可以在连接中设置条件。

//validation checks
else
{
    string connStr = ConfigurationManager.ConnectionStrings["myConnectionString"].ConnectionString;
    MySqlConnection conn = new MySqlConnection(connStr);

    string questionSource = Session["QuestionSource"].ToString();
    string cmdText = "";

    if (questionSource.Equals("S"))
    {
        cmdText += @"SELECT COUNT(*) FROM questions Q
                    JOIN users U
                    ON Q.author_id=U.user_id
                    WHERE approved='Y'
                    AND role=1
                    AND module_id=@ModuleID";

        MySqlCommand cmd = new MySqlCommand(cmdText, conn);
        cmd.Parameters.Add("@ModuleID", MySqlDbType.Int32);
        cmd.Parameters["@ModuleID"].Value = Convert.ToInt32(Session["TestModuleID"]);
    }
    else if (questionSource.Equals("U"))
    {
        cmdText += "SELECT COUNT(*) FROM questions WHERE approved='Y' AND module_id=@ModuleID AND author_id=@AuthorID;";

        MySqlCommand cmd = new MySqlCommand(cmdText, conn);
        cmd.Parameters.Add("@ModuleID", MySqlDbType.Int32);
        cmd.Parameters["@ModuleID"].Value = Convert.ToInt32(Session["TestModuleID"]);
        cmd.Parameters.Add("@AuthorID", MySqlDbType.Int32);
        cmd.Parameters["@AuthorID"].Value = Convert.ToInt32(Session["UserID"]);
    }

    int noOfQuestionsAvailable = 0;
    int noOfQuestionsWanted = Convert.ToInt32(ddlNoOfQuestions.SelectedValue);

    try
    {
        conn.Open();

        noOfQuestionsAvailable = Convert.ToInt32(cmd.ExecuteScalar());

        if (noOfQuestionsAvailable < noOfQuestionsWanted)
        {
            lblError.Text = "There are not enough questions available to create a test.";
        }
        else
        {
            Session["TestName"] = txtName.Text;
            Session["NoOfQuestions"] = ddlNoOfQuestions.SelectedValue;
            Session["QuestionSource"] = rblQuestionSource.SelectedValue;
            Session["TestModuleID"] = ddlModules.SelectedValue;
            Response.Redirect("~/create_test_b.aspx");
        }
    }
    catch
    {
        lblError.Text = "Database connection error - failed to get module details.";
    }
    finally
    {
        conn.Close();
    }
}

Answer 1

在 if 之前声明 cmd

MySqlCommand cmd = new MySqlCommand("",connStr);

在if的每一部分

cmd.CommandText=cmdText;

其他建议：添加

cmd.Parameters.Add("@ModuleID", MySqlDbType.Int32);
cmd.Parameters["@ModuleID"].Value = Convert.ToInt32(Session["TestModuleID"]);

总是在 if 之前，因为它在 if 和 else 部分中以相同的方式使用

Answer 2

您只需将 cmd 的声明移到 if 块之外：

//validation checks
else
{
    string connStr = ConfigurationManager.ConnectionStrings["myConnectionString"].ConnectionString;
    MySqlConnection conn = new MySqlConnection(connStr);

    string questionSource = Session["QuestionSource"].ToString();
    string cmdText = "";

    MySqlCommand cmd; // <-- here
    if (questionSource.Equals("S"))
    {
        cmdText += @"SELECT COUNT(*) FROM questions Q
                    JOIN users U
                    ON Q.author_id=U.user_id
                    WHERE approved='Y'
                    AND role=1
                    AND module_id=@ModuleID";

        cmd = new MySqlCommand(cmdText, conn); // remove MySqlCommand  here
        cmd.Parameters.Add("@ModuleID", MySqlDbType.Int32);
        cmd.Parameters["@ModuleID"].Value = Convert.ToInt32(Session["TestModuleID"]);
    }
    else if (questionSource.Equals("U"))
    {
        cmdText += "SELECT COUNT(*) FROM questions WHERE approved='Y' AND module_id=@ModuleID AND author_id=@AuthorID;";

        cmd = new MySqlCommand(cmdText, conn); // remove MySqlCommand  here
        cmd.Parameters.Add("@ModuleID", MySqlDbType.Int32);
        cmd.Parameters["@ModuleID"].Value = Convert.ToInt32(Session["TestModuleID"]);
        cmd.Parameters.Add("@AuthorID", MySqlDbType.Int32);
        cmd.Parameters["@AuthorID"].Value = Convert.ToInt32(Session["UserID"]);
    }

    int noOfQuestionsAvailable = 0;
    int noOfQuestionsWanted = Convert.ToInt32(ddlNoOfQuestions.SelectedValue);

    try
    {
        conn.Open();

        noOfQuestionsAvailable = Convert.ToInt32(cmd.ExecuteScalar());

        if (noOfQuestionsAvailable < noOfQuestionsWanted)
        {
            lblError.Text = "There are not enough questions available to create a test.";
        }
        else
        {
            Session["TestName"] = txtName.Text;
            Session["NoOfQuestions"] = ddlNoOfQuestions.SelectedValue;
            Session["QuestionSource"] = rblQuestionSource.SelectedValue;
            Session["TestModuleID"] = ddlModules.SelectedValue;
            Response.Redirect("~/create_test_b.aspx");
        }
    }
    catch
    {
        lblError.Text = "Database connection error - failed to get module details.";
    }
    finally
    {
        conn.Close();
    }
}

Answer 3

只需将 MySqlCommand 的声明移到 if/else 块之外，这样您就可以在执行命令的最终尝试中使用它

//validation checks
else
{
    string connStr = ConfigurationManager.ConnectionStrings["myConnectionString"].ConnectionString;
    using(MySqlConnection conn = new MySqlConnection(connStr))
    using(MySqlCommand cmd = conn.CreateCommand())
    {
         // Don't need to associate the command to the connection
         // Already done by the CreateCommand above, just need to set
         // the parameters and the command text

         if (questionSource.Equals("S"))
         {
                cmdText = @"....."
                cmd.CommandText = cmdText;
                ....
         }
         else if (questionSource.Equals("U"))
         {
                cmdText = "........."
                cmd.CommandText = cmdText;
                ....
         } 
         try
         {
              conn.Open();
              noOfQuestionsAvailable = Convert.ToInt32(cmd.ExecuteScalar());
               ....
          }
    }
}

还请注意，您应该使用 using 语句来确保您的连接和您的命令已正确关闭和处置。