MySQL关联left join 条件on与where不同(2)

4.    group by p.pcode order by total desc,p.pid asc 

5.    +---------+-------+------------+-------+ 

6.    | pname | pcode | saletime | total | 

7.    +---------+-------+------------+-------+ 

8.    | 商品1 | AC90 | 2008-09-23 | 2 |  

9.    +---------+-------+------------+-------+ 

10.   select p.pname,p.pcode,s.saletime,count(s.aid) as total from products as p 

11.   left join sales_detail as s on ((s.pcode=p.pcode) and s.saletime in ('2008-09-23','2008-09-24'))  

12.   group by p.pcode order by total desc,p.pid asc 

13.   +---------+-------+------------+---------+ 

14.   | pname | pcode | saletime | total | 

15.   +---------+-------+------------+-------+ 

16.   | 商品1 | AC90 | 2008-09-23 | 2 |  

17.   | 商品2 | DE78 | NULL | 0 |  

18.   | 商品3 | XXXX | NULL | 0 |  

19.   +---------+-------+------------+---------+ 

心得：on中的条件关联，一表数据不满足条件时会显示空值。where则输出两表完全满足条件数据。