登录提示用户不存在

[曾蚊子要学习]

登录代码如下，点击登陆却老是显示用户不存在那个提示，明明数据库里是有这个用户的。。。

using System;
using System.Configuration;
using System.Data;
using System.Data.Common;
using System.Data.SqlClient;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;


public partial class Login : System.Web.UI.Page
{
      
    protected void Page_Load(object sender, EventArgs e)
    {
        
    }

    SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["netexamConnectionString"].ToString());

    protected void login_Click(object sender, EventArgs e)
    {
        int type = Int32.Parse(ddlType.SelectedItem.Value);
        string userid = txtuser.Text.Trim();  //保存登录页面输入的用户名
        string inputpwd = txtpwd.Text.Trim();   //保存登录页面输入的密码
        string pwd = "";
        
        
       try
        {
            string sql = "select * from Users where ID like '" + userid + " '"+"and Role="+type;
            SqlDataAdapter da = new SqlDataAdapter(sql,conn);
            DataSet ds = new DataSet();
            da.Fill(ds);
            if (ds != null && ds.Tables.Count > 0 && ds.Tables[0].Rows.Count > 0 && !ds.Tables[0].Rows[0].IsNull(0)) //判断是否有返回数据，若无，则说明不存在该用户
                {
                    pwd = ds.Tables[0].Rows[0][2].ToString().Trim(); //获取密码
                    if (inputpwd == pwd)  //密码正确登录成功
                    {
                        Session["ID"] = userid;  //在session中记录登录用户ID
                        Session["Type"] = type;  //在session中记录登录用户的用户类型
                        switch (type)
                        {
                            case 0:  // 身份为学生时
                                Response.Redirect("../student/Stu_index.aspx");  //跳到学生主页
                                break;
                            case 1:
                                Response.Redirect("~/Web/Tch_index.aspx");  //跳到教师主页
                                break;
                            case 2:
                                Response.Redirect("~/Web/admin/Ad_index.aspx");
                                break;
                        }

                    }
                    else
                    {
                        Response.Write("<script>alert('密码错误，请检查后重新输入！')</script>");
                    }
                }
              else
                    {
                        Response.Write("<script language='javascript'>alert('该用户不存在或用户名输入错误，请检查后重新输入！')</script>");
                    }
            
            
            }
            
       
       catch (System.Exception ex)
        {
            Response.Write("<script>alert('错误原因："+ex.Message.ToString()+"')</script>");
        }
        finally
        {
        }
    }

   
}

[hjx398]

"select * from Users where ID ='" + userid + " '"+"and Role="+type;