public String getSingleEntry(String username) {
        Cursor cursor = db.query("credentials", null, " USERNAME=?", new String[] {username}, null, null, null);
        if (cursor.getCount() < 0)//1->0,nomizw to 0 simenei oti brike 1
        {
            cursor.close();
            return "NOT EXIST";
        }
        if(cursor.getColumnCount() <0)//1->0,nomizw to 0 simenei oti brike 1
        {
            cursor.close();
            return "NOT EXIST";//apla to vala gia to message
            //Den pire ka8olou data an kai perase to Where Clause
        }
        if(cursor.moveToFirst())//ama kataferi na paei sthn arxh
        {
            String password = cursor.getString(cursor.getColumnIndex("PASSWORD"));//to la8os ginotan EDW,den ebriske to column me onoma "PASSWORD"
            cursor.close();
            return password;
        }
        else {
            cursor.close();
            return "NOT EXIST";
        }//den borese na paei sthn arxh
    }