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
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)。
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=MyISAMAUTO_INCREMENT=1DEFAULTCHARSET=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=MyISAMDEFAULTCHARSET=utf8AUTO_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');
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 | 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。
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