现有两个表，商品表(products)和sales_detail(销售记录表)。如下图:

products:
      pid pname pcode
    1   商品1   AC90
    2   商品2 DE78
     3   商品3    XXXX

sales_detail:
    aid pcode saletime
    1   AC90 2008-09-22
    2   DE78 2008-09-22
    3   AC90 2008-09-23
    4   AC90 2008-09-24

按商品在某个时间段内的销售量来排行，比如我想统计23－24号这两天的销售数量并排行。（注：DE78这个商品在这两天没有销售，但是也要显示出来，只是数量为0）。

-- 表的结构 `products`
--
CREATE TABLE `products` (
   `pid` int(3) NOT NULL auto_increment,
   `pname` varchar(20) NOT NULL,
   `pcode` varchar(20) NOT NULL,
   PRIMARY KEY   (`pid`)
) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
--
-- 导出表中的数据 `products`
--
INSERT INTO `products` (`pid`, `pname`, `pcode`) VALUES (1, '商品1', 'AC90'),
(2, '商品2', 'DE78'),
(3, '商品3'. 'XXXX');
-- --------------------------------------------------------
--
-- 表的结构 `sales_detail`
--
CREATE TABLE `sales_detail` (
   `aid` int(3) NOT NULL auto_increment,
   `pcode` varchar(20) NOT NULL,
   `saletime` date NOT NULL,
   PRIMARY KEY   (`aid`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1;
--
-- 导出表中的数据 `sales_detail`
--
INSERT INTO `sales_detail` (`aid`, `pcode`, `saletime`) VALUES (1, 'AC90', '2008-09-22'),
(2, 'DE78', '2008-09-22'),
(3, 'AC90', '2008-09-23'),
(4, 'AC90', '2008-09-24');

区别两条sql语句：

select p.pname,p.pcode,s.saletime,count(s.aid) as total from products as p
left join sales_detail as s on (s.pcode=p.pcode)
where s.saletime in ('2008-09-23','2008-09-24')
group by p.pcode order by total desc,p.pid asc

+---------+-------+------------+-------+
| pname   | pcode | saletime   | total |
+---------+-------+------------+-------+
| 商品1   | AC90 | 2008-09-23 |     2 |
+---------+-------+------------+-------+

select p.pname,p.pcode,s.saletime,count(s.aid) as total from products as p
left join sales_detail as s on ((s.pcode=p.pcode) and s.saletime in ('2008-09-23','2008-09-24'))
group by p.pcode order by total desc,p.pid asc

+---------+-------+------------+---------+
| pname   | pcode | saletime   | total |
+---------+-------+------------+-------+
| 商品1   | AC90 | 2008-09-23 |     2 |
| 商品2   | DE78 | NULL       |     0 |
| 商品3   | XXXX   | NULL       |     0 |
+---------+-------+------------+---------+

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。