MySql“选择位置"和C# [英] MySql "Select Where" and C#

问题描述

如何从"Select Where"语句中读取返回值，每次运行时，标签中都没有出现返回值，也没有语法错误.

How can i read the return value from "Select Where" statement , every time i run no return value appear in the label, and no syntax error.

command.CommandText = "select product_price from product where product_name='"+x+"';";
            connection.Open();
            Reader = command.ExecuteReader();
            while(Reader.Read()){


            Price_label.Content = "" + Reader.GetString(0);

            }
            connection.Close();

推荐答案

如果product_price列在MySQL中不是TEXT类型，则Reader.GetString(0)将(取决于Oracle如何实现读取器)抛出Exception或返回空字符串.我认为后者正在发生.

If the product_price column is not of type TEXT in MySQL, the Reader.GetString(0) will (depending on how the reader was implemented by Oracle) throw an Exception or return an empty string. I would think the latter is happening.

通过DataReader检索值要求您知道数据类型.您不能简单地为每种类型的字段读取字符串.例如，如果数据库中的字段是Integer，则需要使用GetInt32(...).如果它是DateTime，请使用GetDateTime(...).在DateTime字段上使用GetString无效.

Retrieving the value through a DataReader requires you to know the data type. You can not simply read a string for every type of field. For example, if the field in the database is an Integer, you need to use GetInt32(...). If it is a DateTime use GetDateTime(...). Using GetString on a DateTime field won't work.

编辑
这就是我编写此查询的方式:

EDIT
This is how I'd write this query:

using (MySqlConnection connection = new MySqlConnection(...))
{
    connection.Open();
    using (MySqlCommand cmd = new MySqlCommand("select product_price from product where product_name='@pname';", connection))
    {
        cmd.Parameters.AddWithValue("@pname", x);
        using (MySqlDataReader reader = cmd.ExecuteReader())
        {
            StringBuilder sb = new StringBuilder();
            while (reader.Read())
                sb.Append(reader.GetInt32(0).ToString());

            Price_label.Content = sb.ToString();
        }
    }
}

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