使用SqlCommand选择数据，如果数据不存在，则插入数据，有时数据可能为空

using System;
using System.Data.SqlClient;

public class Program
{
    public static void Main()
    {
        string connectionString = "your_connection_string";
        string selectQuery = "SELECT * FROM your_table WHERE your_condition";
        string insertQuery = "INSERT INTO your_table (column1, column2) VALUES (value1, value2)";

        using (SqlConnection connection = new SqlConnection(connectionString))
        {
            SqlCommand selectCommand = new SqlCommand(selectQuery, connection);
            connection.Open();
            SqlDataReader reader = selectCommand.ExecuteReader();

            if (reader.HasRows)
            {
                // 数据存在，可以进行其他操作
                Console.WriteLine("数据已存在");
            }
            else
            {
                // 数据不存在，执行插入操作
                reader.Close();
                SqlCommand insertCommand = new SqlCommand(insertQuery, connection);
                insertCommand.ExecuteNonQuery();
                Console.WriteLine("数据已插入");
            }

            connection.Close();
        }
    }
}