Session id redirect issue

Question

i have login form,in which i am loggin from User Name and password ,at the befhalf of user name and password i am retreiving user id in session,but when page is redirect then just first ID of table is being retreived every time.

 

below is c# code 

using (SqlCommand cmd = new SqlCommand("Validate_User"))
          {


              cmd.CommandType = CommandType.StoredProcedure;
              cmd.Parameters.AddWithValue("@Username", username.Text.Trim());
              cmd.Parameters.AddWithValue("@Password", password.Text.Trim());
              cmd.Connection = con;
              con.Open();
              SqlDataAdapter sda = new SqlDataAdapter(cmd);
              DataTable dt = new DataTable();
              sda.Fill(dt);
              if (dt.Rows.Count > 0)
              {
                  // lblmsg.Text = "Login Successful";
                  Session["S_ID"] = dt.Rows[0]["S_ID"].ToString();

                  Response.Redirect("issue_std_Books.aspx");
              }
              else
              {
                  // lblmsg.Text = "Invalid UserName/Password";
              }
              con.Close();
              con.Dispose();
          }
      }

SQl store procedure

 

ALTER PROCEDURE [dbo].[Validate_User]
@Username NVARCHAR(20),
      @Password NVARCHAR(20)
AS
BEGIN
      SET NOCOUNT ON;
      DECLARE @UserId INT

      SELECT @UserId = S_ID
      FROM tbl_Student WHERE [S_Name] = @Username AND t_pwd = @Password

      IF @UserId  IS NOT NULL
      BEGIN
            SELECT S_ID FROM tbl_Student
      END
      end