creating a primary key for already existing code

here is my code... i am trying to create primary key... pls anyone help me...

package com.example.login;

import android.app.Activity;
public class Addstudents extends Activity implements OnClickListener {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_addstudents);

        Button save = (Button) findViewById(R.id.less60);
         save.setOnClickListener(this);

        Button clear = (Button) findViewById(R.id.less75);
        clear.setOnClickListener(this);

        Button back = (Button) findViewById(R.id.less85);
        back.setOnClickListener(this);


        SQLiteDatabase db;
        db = openOrCreateDatabase( "Student.db" , SQLiteDatabase.CREATE_IF_NECESSARY        , null          );
        try {
            final String StudentDetails = "CREATE TABLE IF NOT EXISTS StudentDetails ("
                    + "REG_NO," 
                    + "ROLL_NO,"
                    + "NAME,"
                    + "SEM,"
                    + "CONTACT_NO);";
            db.execSQL(StudentDetails);

          //  Toast.makeText(Addstudents.this, "table created ", Toast.LENGTH_LONG).show();



        }
        catch (Exception e) {
            Toast.makeText(Addstudents.this, "ERROR "+e.toString(), Toast.LENGTH_LONG).show();  
    }
    }

        public void onClick(View view){

            SQLiteDatabase db;


             if(view.getId()==R.id.less60)
             {
                try
                {
                 EditText e1 = (EditText) findViewById(R.id.code1);

                 EditText e2 = (EditText) findViewById(R.id.rollno);

                 EditText e3 = (EditText) findViewById(R.id.name1);
                 String s3=e3.getText().toString();

                 EditText e4 = (EditText) findViewById(R.id.editText2);

                 EditText e5 = (EditText) findViewById(R.id.phno);

                 String sql =
                            "INSERT or replace INTO StudentDetails (" +
                            "Reg_no, " +
                            "Roll_no, " +
                            "Name, " +
                            "Sem,"+
                            "Contact_no)"+
                            "VALUES('"+e1+"','"+e2+"','"+s3+"','"+e4+"','"+e5+"')" ;       

        db = openOrCreateDatabase( "Student.db" , SQLiteDatabase.CREATE_IF_NECESSARY        , null          );

       db.execSQL(sql);

        }
        catch (Exception e) {
    Toast.makeText(Addstudents.this, "ERROR "+e.toString(), Toast.LENGTH_LONG).show(); 
        }            
      AlertDialog alertDialog = new AlertDialog.Builder(Addstudents.this).create();
       alertDialog.setTitle("Alert Dialog");

        alertDialog.setMessage("students details have successfully added to the datebase");

        alertDialog.setButton("OK", new DialogInterface.OnClickListener() {
          public void onClick(DialogInterface dialog, int which) {
          // Write your code here to execute after dialog closed
          // Toast.makeText(getApplicationContext(), "You clicked on OK", Toast.LENGTH_SHORT).show();
                       }
                });
                    alertDialog.show();     


        }
             else if(view.getId()==R.id.less75)
             {
                 Intent stu = new Intent(this, Addstudents.class);
                   startActivity(stu);

             }
             else if(view.getId()==R.id.less85)
             {
                 Intent att = new Intent(this, Attendence.class);
                   startActivity(att);

             }      
            }

}

please anyon help me to create primary key for regno and rollno