Insert to DB if not exist

I tried to insert row to DB if it doesn't exist by email:

public boolean AddUserIfNotExist(String email, String password, String username) {

    SQLiteDatabase db = m_helper.getWritableDatabase();


    //String Query = "INSERT OR IGNORE INTO USERS(email, password, username) 
    values('" + email + "','" + password + "','" + username + "');";

    String Query = "INSERT INTO USERS(email, password, username) " +
            "SELECT '" + email + "','" + password + "','" + username + "' " 
            + "WHERE NOT EXISTS(SELECT 1 FROM USERS WHERE email = '" + email + "');";

    Cursor cursor = db.rawQuery(Query, null);

    if(cursor.getCount() <= 0) {
        return false;
    }
    return true;
}

Why it doesn't work? cursor.getCount always returns false.