Comparing two rows in the same table in sql server
try
{
   using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["STRING"].ConnectionString))
   {
     using (SqlCommand cmd = new SqlCommand("dbo.Res_User", con))
     {
       cmd.CommandText = "INSERT INTO Res_User(username, password, key_pin) SELECT '" + username + "' , dbo.fnEncDecRc4('" + pin + "','" + password + "'), '" + pin + "'";
       con.Open();
       cmd.ExecuteNonQuery();

       MessageBox.Show("Added", "Information", MessageBoxButtons.OK);

       cmd.CommandText = "SELECT password FROM Res_User WHERE username = @username";
       cmd.Parameters.AddWithValue("@username", username);
       cmd.ExecuteNonQuery();

       using (SqlDataReader reader = cmd.ExecuteReader())
       {
           if (reader.HasRows)
           {
               while (reader.Read())
               {
                    MessageBox.Show(reader["password"].ToString(), "Information", MessageBoxButtons.OK);
               }
           }
       }

try
{
   using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["STRING"].ConnectionString))
   {
     using (SqlCommand cmd = new SqlCommand("dbo.Res_User", con))
     {
       cmd.CommandText = "INSERT INTO Res_User(username, password, key_pin) SELECT '" + username + "' , dbo.fnEncDecRc4('" + pin + "','" + password + "'), '" + pin + "'";
       con.Open();
       cmd.ExecuteNonQuery();

       MessageBox.Show("Added", "Information", MessageBoxButtons.OK);

       cmd.CommandText = "SELECT password FROM Res_User WHERE username = @username AND key_pin = @pin AND password = dbo.fnEncDecRc4(@pin, @password)";
       cmd.Parameters.AddWithValue("@username", username);
       cmd.Parameters.AddWithValue("@pin", pin);
       cmd.Parameters.AddWithValue("@password", password);
       cmd.ExecuteNonQuery();

       using (SqlDataReader reader = cmd.ExecuteReader())
       {
           if (reader.HasRows)
           {
               //successfully validated.
           }
       }