mysql - sql 分组查询问题。。

Question

表1

表2

查询语句和结果

结果只显示3条数据，我要显示product表的所有数据，怎么回事？

Answer 1

Please write like this, the reason for this is improper use of group by
select p.id,p.productname,p.productcode,p.price,l.num from p
left join (select pid, sum(num) as num from wlms_stock group by pid) l on p.id = l.pid

Answer 2

group by should be placed after line 29