mysql

group by 功能为分组，分组后可用于统计使用

查询子类个数，可通过自连接实现

select a.id,count(*) as children_num from test as a left join test as b on a.id = b.parent_id

group by a.id

转载于:https://www.cnblogs.com/gaocy/p/8549168.html