/**
     * 功能：判断该表是否已存在于数据库中
     * @param "表名"
     */
    public boolean IsExist(String table){
        boolean exist = false;
        String sql = "select * from sqlite_master where name="+"'"+table+"'";
        Cursor cursor = db.rawQuery(sql,null);
        if(cursor.getCount()!=0){
            exist = true;
        }
        return exist;
    }