如何在文本框中顯示SQL查詢結果？

Question

我有以下代碼：

dbConnection cn = new dbConnection(); 
SqlCommand cmd = new SqlCommand(); 
protected void dropdown_student_SelectedIndexChanged(object sender, EventArgs e) 
{ 
    string StudentGUID = dropdown_student.SelectedValue; 
    cn.con.Open(); 
    cn.cmd.Connection = cn.con; 
    cn.cmd.CommandText = "select SUM(Marks) AS 'Total' from Marksheet where StudentGUID = " + StudentGUID + " "; 
    dr = cn.cmd.ExecuteReader(); 
    if (dr.HasRows) 
    {    
     while (dr.Read()) 
     { 
      textbox_total.Text = dr["Total"].ToString(); 
     } 
     cn.con.Close(); 
    } 
} 

我要顯示在文本框中佔總分的價值，但它不工作。任何人都可以將我指向正確的方向嗎？

Answer 1

嘗試這樣：

// define your query upfront - using a PARAMETER! 
string query = "SELECT SUM(Marks) FROM dbo.Marksheet WHERE StudentGUID = @StudentID;"; 

// put the SqlConnection and SqlCommand into using blocks  
using (dbConnection cn = new dbConnection()) 
using (SqlCommand cmd = new SqlCommand(query, cn)) 
{ 
    // define the parameter value 
    cmd.Parameters.Add("@StudentID", SqlDbType.UniqueIdentifier).Value = dropdown_student.SelectedValue; 

    cn.Open(); 

    // use ExecuteScalar if you fetch one row, one column exactly 
    object result = cmd.ExecuteScalar(); 
    cn.Close(); 

    if(result != null) 
    { 
     int value = (int)result; 
     textbox_total.Text = value.ToString(); 
    } 
} 

Answer 2

嘗試把單引號周圍的GUID變量SQL查詢。

cn.cmd.CommandText = "select SUM(Marks) AS 'Total' from Marksheet where StudentGUID = '" + StudentGUID + " '"; 

Answer 3

您的SQL查詢有誤。試試如下：

string query = string.Format("SELECT SUM(Marks) FROM dbo.Marksheet WHERE StudentGUID = '{0}';", StudentID); 

總是試圖遵循這種標準的做法，你永遠不會失敗。