两张表联合查询，并统计其中一张表在关联条件下的数据个数

select a.dfsname, count(b.dfsid) as instnum from tbl_dfslist a left join tbl_instance b on a.id = b.dfsid group by a.id;

（其中tbl_dfslist.id = tbl_instance.dfsid）