MySQL關聯left join 條件on與where不同

以下的文章主要講述的是MySQL關聯left join 條件on與where 條件的不同之處，我們現在有兩個表，即商品表(products)與sales_detail(銷售記錄表)。我們主要是通過這兩個表來對MySQL關聯left join 條件on與where 條件的不同之處進行講述。

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 

某個時間段內的銷售量來排行，比如我想統計23－24號這兩天的銷售數量并排行。（注：DE78這個商品在這兩天沒有銷售，但是也要顯示出來，只是數量為0）。

MySQL關聯left join條件on和where條件的區別表的結構 `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; 

導入表中的數據 `products`

INSERT INTO `products` (`pid`, `pname`, `pcode`) VALUES 
(1, '商品1', 'AC90'),  
(2, '商品2', 'DE78'),  
(3, '商品3', 'XXXX'); 

表的結構 `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; 

表中的數據 `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關聯left join條件on和where條件的區別區別兩條sql語句：

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中的條件關聯，一表數據不滿足條件時會顯示空值。where則輸出兩表完全滿足條件數據。