MySQL关联left join 条件on与where不同

以下的文章主要讲述的是MySQL关联left join 条件on与where 条件的不同之处，我们现在有两个表，即商品表(products)与sales_detail(销售记录表)。我们主要是通过这两个表来对MySQL关联left join 条件on与where 条件的不同之处进行讲述。

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 

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）。

MySQL关联left join条件on和where条件的区别表的结构 `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; 

导入表中的数据 `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; 

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'); 

MySQL关联left join条件on和where条件的区别区别两条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   |
+-------+-------+------------+
| 商品1 | AC90  | 2008-09-23 |
+-------+-------+------------+

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。