c# - How to access only for admin to login the page? -

i want staff role a (a=administrator) in staff table can access administrator module. want mary tan can access login while rest of staff cannot access.

here output:

click image

database:

click db table

here's code:

 sqlconnection conn = null;         sqlcommand cmd = null;         string connectionstring = null;         string staffname = null;         protected void page_load(object sender, eventargs e)         {          }          public bool checkvaliduser(string username, string password)         {             bool valid = false;             sqldatareader dr = null;              connectionstring = configurationmanager.connectionstrings["leavemanagementcs"].connectionstring;              string sql = "select * staff username=@username , password=@pwd";              try             {                 conn = new sqlconnection(connectionstring);                  cmd = new sqlcommand(sql, conn);                  cmd.parameters.addwithvalue("@username", username);                 cmd.parameters.addwithvalue("@pwd", password);                  conn.open();                  dr = cmd.executereader();                  if (dr.read())                 {                     staffname = dr["staffname"].tostring();                     valid = true;                 }                 else                 {                     lbloutput.text = "there error logging in. please check username or password.";                 }                 dr.close();             }             catch (exception ex)             {                 lbloutput.text = "error message: " + ex.message;             }                         {                 if (conn != null)                     conn.close();             }             return valid;         }          protected void tblogin_click(object sender, eventargs e)         {             bool validuser = checkvaliduser(tbusername.text, tbpassword.text);              if (validuser)             {                 session["staffname"] = staffname;                  formsauthentication.setauthcookie(staffname, false);                  formsauthentication.redirectfromloginpage(staffname, false);             }             else             {                  lbloutput.text = "invalid user. please try again.";             }         } 

change sql query this:

string sql = "select * staff username=@username , password=@pwd , role=n'a';