我正在尝试从类别表中获取顶级父ID:+-------------+----------------------+--------+|category_id|name|parent|+-------------+----------------------+--------+|1|ELECTRONICS|NULL||2|TELEVISIONS|1||3|TUBE|2||4|LCD|2||5|PLASMA|2||6|PORTABLEELECTRONICS|1||7|MP3PLAYERS|6||8|FLASH|7||9|CDPLAYERS|6||10|2WAYRADIOS|6|+------