Question

早上好，我正在开发一个代码，可以让我从某个专栏的AVG中脱离我的数据库。问题是我没有得到它，我认为问题是查询，但我不知道如何解决它。我需要帮助，谢谢。

以下是代码：

String connectionString =
       "Data Source=localhost;" +
       "Initial Catalog=DB_SACC;" +
       "User id=sa;" +
       "Password=1234;";

        SqlConnection connection = new SqlConnection(connectionString);

        SqlCommand cmd = new SqlCommand();

        string textt = " USE [DB_SACC] SELECT AVG (Total_Divida) FROM t_pagamentos";

        cmd.CommandText = textt;

        connection.Open();

        cmd.Connection = connection;

        cmd.CommandType = CommandType.Text;

        cmd.ExecuteNonQuery();

        if (textt == null)
        {
            MessageBox.Show("nothing");
        }
        else
        {
            TextBox3.Text = textt;
        }

Answer 1

请改用cmd.ExecuteScalar（）方法：

decimal average = (decimal) cmd.ExecuteScalar();

cmd.ExecuteNonQuery();只返回影响的行数，你想要的是读取SELECT语句的结果集。

我也会从你的USE [DB_SACC]语句中删除SELECT，因为你在连接字符串中定义了数据库名称。

修改

您的代码应如下所示：

string textt = "SELECT AVG (Total_Divida) FROM t_pagamentos"; cmd.CommandText = textt; connection.Open(); cmd.Connection = connection; cmd.CommandType = CommandType.Text; decimal average = (decimal) cmd.ExecuteScalar(); if (textt == null) { MessageBox.Show("nothing"); } else { TextBox3.Text = average.ToString(); }

编辑2：

try { string textt = "SELECT AVG (Total_Divida) FROM t_pagamentos"; cmd.CommandText = textt; connection.Open(); cmd.Connection = connection; cmd.CommandType = CommandType.Text; decimal average = (decimal)cmd.ExecuteScalar(); TextBox3.Text = average.ToString(); } catch(Exception ex) { // log your exception here... MessageBox.Show("nothing"); }

编辑3：

根据您最近的评论，试试这个

string connectionString = "Data Source=localhost; Initial Catalog=DB_SACC; User id=sa Password=1234;"; decimal? average; try { using (SqlConnection connection = new SqlConnection(connectionString)) { using (SqlCommand cmd = new SqlCommand()) { string textt = "SELECT AVG (Total_Divida) AS 'AVG_DIVIDA' FROM t_pagamentos"; cmd.CommandText = textt; connection.Open(); cmd.Connection = connection; cmd.CommandType = CommandType.Text; using (DataReader reader = cmd.ExecuteReader()) { while (reader.Read()) { average = decimal.parse(reader["AVG_DIVIDA"].ToString()); break; } } } } TextBox3.Text = average.HasValue ? average.ToString() : "Unknown error occurred"; } catch (Exception ex) { MessageBox.Show("Unable to retrieve the average, reason: " + ex.Message); }

注意：不希望使用DataReader从数据库中获取单个值。由于您在评论中提到的错误，我建议这样做。

如果您收到任何SQL异常，请尝试在SQL Server上运行此语句作为独立测试。

USE DB_SACC GO SELECT AVG (Total_Divida) AS 'AVG_DIVIDA' FROM t_pagamentos GO

如果您在执行T-SQL语句时仍遇到任何错误，请将其作为另一个问题发布。

Answer 2

如果您从数据库中请求单个值，请使用ExecuteScalar - ExecuteNonQuery仅返回更新/插入语句中使用的受影响行数

USE [DB_SACC]

"Initial Catalog=DB_SACC;"
添加using以避免打开连接

代码：

string connectionString =  "Data Source=localhost;Initial Catalog=DB_SACC;User id=sa;Password=1234;";

using (SqlConnection connection = new SqlConnection(connectionString))
{
    string textt = "SELECT AVG (Total_Divida) FROM t_pagamentos";
    using (SqlCommand cmd = new SqlCommand(textt, connection))
    {
        connection.Open();
        var result =  cmd.ExecuteScalar(); //write the result into a variable

        if (result == null)
        {
            MessageBox.Show("nothing");
        }
        else
        {
            TextBox3.Text = result.ToString();
        }
    }
}

c＃中的AVG QUERY

2 个答案: