Already set level 3, what is level 3!? Just connect it yourself twice.

select c1.name "一级分类", c2.name "二级分类", c3.name "三级分类"
from category c1
inner join category c2 on c1.id=c2.parent_id
inner join category c3 on c2.id=c3.parent_id
where c1.parent_id=0 #最高级;

This way you can get the watch you want.

php unlimited classification database design

The database is divided into three tables: first_level, second_level, third_level

first_level: id, name;
second_level: id, parent_id, name    // second_level.parent_id = first_level.id
third_level: id, parent_id, name    // third_level.parent_id = second_level.id

First traverse the level 3 array and throw it into the level 2 sub-item array, similar to Infinitus going in reverse

What you masters want is php code
In fact, it is a reordering problem. The second and third level classifications included under the first level classification,

static public function toLevel($cate, $delimiter = '|——', $parent_id = 0, $level = 0) {

    $arr = array();
    foreach ($cate as $v) {
        if ($v['parent_id'] == $parent_id) {
            $v['type'] = $level + 1;
            $v['delimiter'] = str_repeat($delimiter, $level);
            $arr[] = $v;
            $arr = array_merge($arr, self::toLevel($cate, $delimiter, $v['cate_id'], $v['type']));
        }
    }

    return $arr;

}
这个是我的无限极分类,你的三级分类和我类似,只要渲染前段的时候注意就好了

Using recursive queries in SQL