c# - c# 中的 AVG 查询

Question

早上好，我正在开发一个代码，可以让我在 sql 中退出数据库，即某个列的 AVG。问题是我不明白，我认为问题出在查询上，但我不知道如何解决。我需要帮助，谢谢。

代码如下:

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

• 如果您从数据库请求单个值，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] 在您的查询中不是必需的，因为您定义了 "Initial Catalog=DB_SACC;"
• 添加使用以避免打开连接

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();
        }
    }
}